A guide about bosses and how to summon them

Are you curious how to summon all the 16 bosses in the game? What initiates events? This will tell you both the normal progression and how to refight them again with summon items!

Eye of Chtulhu

"An evil presence is watching you"



Randomly appears at the start of night if not yet defeated if the player has used 5 or more Life Crystals to increase thier max hp(200) or if more than 2 NPCs are living in houses. This chance is low though so you might want to create a:

Suspicious Looking Eye:These can be dropped by enemies and found in chests. Or you can craft them yourself at a Demon altar /Crimson altar with 6 Lens(es). Summoned during night. (Lenses are dropped by demon eyes during night)

Eater of Worlds

"Screams echo around you"



Only available in a Corruption world.
In the underground of the corruption you will find several purple glowing orbs, if you break 3 of these with any Hammer the Eater of Worlds will spawn.
Each 3 shadow orb you break will spawn an Eater of Worlds making it easy to refight.

Worm FoodIf your world has run out of Shadow Orbs or if you are in a crimson world then you can craft the Worm Food at a Demon / Crimson altar using 15 Rotten chunk and 30 Vile powder. (Rotten chunk are ranomly dropped by the enemies in the corruption biome and vile powder can be made at an alchemy station(table w Bottle) from Vile Mushrooms found in the corruption biome). Summoned in corruption. Vile Powder is also sold by the Dryad during a Blood Moon

Brain of Cthulhu

"A horrible chill runs down your spine"



Only available in a Crimson world
In the underground of the crimson you will find several red glowing anatomical heart, if you break 3 of these with any Hammer the Brain of Cthulhu will spawn.
Each 3 Crimson Hearts you break will spawn a Brain of Cthulhu making it easy to refight.

Bloody SpineIf your world has run out of Crimson Hearts or if you are in a corruption world then you can craft the Bloody Spine at a Demon / Crimson altar using 15 Vertebrae and 30 Vicious powder. (Vertebrae are ranomly dropped by the enemies in the crimson biome and vicious powder can be made at an alchemy station(table w Bottle) from Vicious Mushrooms found in the crimson biome). Summoned in crimson. Vicious Powder is also sold by the Dryad during a Blood Moon

Skeletron

"My master cannot be summoned under the light of day."



Either East or West of your starting point (opposite of the jungle) is a castle looking biome called the"Dungeon", at the entrance of this dungeon you will find an NPC called "Old man", speak with the old man during the night and choose the option "Curse" to kill the Old Man and summon Skeletron instead. Killing Skeletron grants access to the dungeon. If you fail to kill Skeletron, the Old man will return at the start of the next day.

Clothier Voodoo Doll:The old man is no longer found at the dungeon after successfully defeating skeletron. But the enemies in the dungeon(Angry Bones, Dark Casters) have a small chance to drop an Accessory called the Clothier Voodoo Doll.
Equipping this Accessory allows you to damage the Clothier NPC. Skeletron will be summoned into the world once more, but only during night, if you damage the clothier enough to kill him with this accessory equipped. The accessory is not destroyed in the process and can be reused once the clothier returns.

King Slime

A completely optional boss that can appear in 3 different ways:

• 1. Random spawn at the edges of the world:
  Whenever you kill a Blue or Green slime in a Purity biome (grass) there is a small chance that the King slime is summoned afterwards. This chance is quite low however. The player must be at the edge of the world, near the Ocean biome, or at least past the dungeon / jungle to trigger this.
  
  
• 2. Silme Rain Event:
  More on this under "Pre-hardmode events".
  The Slime Rain event randomly happens during any time of the day if the players has used at least 2 Heart crystals to increase their max HP(14). Only slimes of different types will spawn during the event and King Slime is summoned once a total of 150 slimes is killed.
  
  
• 3. Slime Crown:The easiest way to summon King Slime is to use a Slime Crown, this is crafted with a Gold Crown, or a Platinum Crown and 20 Gel at a Demon / Crimson Altar. To craft the Gold / Platinum Crowns you need 5 Gold / Platinum Bars(20 ore) and 1 Ruby.

Deerclops

"Hailing from a distant land, this cyclopic monstrosity lurks in cold places, wreaking havoc on those who disturb it."

Added in the 2021 update with the Don't Starve Together Crossover.
Deerclops will spawn in the Snow Biome (tundra) during a blizzard weather at midnight if the player has at least 200 max hp.

Deer Thing: Can be used to summon Deerclops in the Snow biome.Crafted with 3 Flinx Fur , 1 Lens and 5 Demonite/Crimtane Ore at the Demon/Crimson Altar

Queen bee

Since 1.2 there are several "Hornet's nests" spawned in the underground jungle biome. They are easily spotted in contrast to the brown / green palette of the jungle with its yellow and orange tints. Finding the actual hive is sometimes a bit tricky but if you spot honey blocks, or pools of honey in the jungle - that's usually a sign that there is a hive nearby.
Inside the hive is an object called "Larva" that looks a bit like a piece of amber. Any type of object that passes or touches the Larva will break it and summon Queen bee, even hooks.

Note: Queen Bee enrages outside of the Jungle biome since 1.4

Abeemination:The recipe of this item was luckily reduced to just 4 items and no longer requires a Demon / Crimson Altar to craft. What you need is 1 Stinger, dropped by the Hornets in the underground jungle. 5 Hive, mined from the Hornet Nests. 5 Honey Blocks, that are created when Water meets Honey or Honey meets water. And lastly 1 Bottled honey, that you craft by standing near a pool of honey with a Bottle in your inventory. This will summon Queen Bee when used as long as you are in the jungle biome.

Wall of Flesh

"The guide has been killed"



This boss exists to challenge players and test if they are ready for hardmode.

And therefore there is no natural way for it to spawn except finding a:

Guide Voodoo Doll:
At the lowest point of the world known as "The underworld" (or hell) you will find flyin Demons among the enemies. Some of these Demons sometimes spawn with a different sprite, with something hanging underneath them.


Killing one of these demons makes them drop an item known as the Guide Voodoo Doll. These "Voodoo Demons" only spawns once The Eater of Worlds or Brain of Cthulhu is defeated though(has this been changed?). Unlike the Clothier Voodoo Doll it is not enough to kill the guide with it equipped. Instead you must destroy the item by making it touch the lava of the underworld layer.
Doing this kills the guide and summons the Wall of Flesh at the end of the world (so if you're to the east it will spawn at the east end, if you're to the west it will spawn at the west end)

To resummon the Wall of Flesh after defeating it you can just repeat the process. But, the guide must be alive to be sacrifced.
Note that Wall of Flesh can be accidently summoned if a Demon is killed above the lava and drops the Voodoo Doll upon death making it break in the lava and killing the guide. (Reminded by Thomasgdl1)

Pre hardmode Events

Blood Moon:

Blood Moons can randomly occur at night when you have reached 120 max hp (used 1 life crystal). The screen tints red and a message will signal the event's start. This will increase all monster spawn rates and can be used to improve the farming of some monsters with low spawn rate. Some unique enemies will spawn during the blood moon such as blood Zombies, Dripplers, The Groom and The Bride. Also in hardmode, The Clown will spawn (which used to be horrible).
Also, some NPCs will sell new items during a Blood Moon, especially Corruption themed ones.
Added in 1.4 is more enemies and a mini boss connected to fishing during a blood moon, including two difficult monsters or "mini bosses" in hardmode.

Bloody Tear: An item added in 1.4 that will activate a Blood Moon during nights.
It is dropped by the Wandering Eye Fish, Zombie Merman, Blood Zombie, Drippler, Clown, Blood Eel, and Hemogoblin Shark, The Groom, The Bride, and Dreadnautilus. (So all Blood Moon enemies pretty much).
- Brides and Grooms that spawn in Graveyards can now drop Bloody Tears even if it isn't a Blood Moon

Silme Rain:

The Slime Rain event randomly happens during any time of the day if the players has used at least 2 Heart crystals to increase their max HP (140).
In an Expert world there are apparently no criterias that needs to be reaches as some people have noticed their second-third day to be a slime filled massacre. The chance is really low and some players don't encounter it at all. Official information states that slime rain is more likely to occur if the King Slime has not been defeated in the world and that the chance is reduced once the player reaches Hardmode.
Only slimes of different types will spawn during the event and King Slime is summoned once a total of 150 slimes is killed. The event ends as King Slime is defeated or a whole ingame day passes. Note that there is no progress bar for this event.

Some players have reported that it seems to occur more often if you stay at the edges of the world and that once you reach 400hp the chance is lower. There is however no official information to support this.

Goblin Invasion:

The Goblin invasion only has a chance to happen when 2 criterias are fulfilled: 1. You broke at least one Shadow orb / Crimson heart in the world, 2. You have used at least 5 Heart crystals to increase your max hp(140). This event is triggered at the start of a new day and the goblins arrive from the direction mentioned in the message. You don't need to kill all the goblins that spawn to complete the event but some people have reported that leaving some makes the progress bar stay and the music play. There is approximately 120 goblins during the event. During hardmode a new enemy called the Gobling Summoner is added that can drop rather good weapons. The chance of a Goblin invasion to happen is lower in hardmode.

Goblin Battle Standard:If you want to summon the goblin invasion on your own then you can craft a Goblin Battle Standard at a Loom using 5 Wood and 10 Tattered Cloth. Tattered Cloth are dropped by the rare enemy "Goblin Scout" sometimes found at the edge of the world, near the ocean biome. They will drop a rather low ammount of Tattered Cloth(2-4) so you might need to kill a few. This can be used at any time. Lifeform Analyser will spot the Goblin Scouts.

Old One's Army 1

New in 1.3.4

This event is a collaboration with Dungeon Defenders 2 and features gameplay, enemies and items inspired from that game.
This has 3 different versions but I'm adding it all here for convinience.

To start it you first need to find the Tavernkeep NPC who spawns in the overworld after you defeat EoW/BoC and then purchase an:
Eternia Crystal Stand for 1g. The Eternia Crystal stand is a furniture that needs to be placed in your arena. It needs some space (60 free tiles on each side of it and at least 12 tiles upwards) to place. Once placed you need to click on the furniture with an Eternian Crystal in your inventory to activate, like how you interact with other furniture. If there isn't enough room you'll get an error message.

Eternian Crystal: Costs 25s(1g then 3g in hardmode)and is used with the stand to activate the event.
Once activated you cannot place or break blocks or anything else during the event.

In normal mode the event consists of 5 waves and in hardmode it is 7 waves instead. What the waves include depend on your progress in the world.
You need to protect the Eternian Crystal from being destroyed to win by defeating all the enemies that spawn.

• In normal there are 5 waves and a mini- boss called Dark Mage.
  
• In hardmode after defeating at least one mecha boss you will fight Ogre in wave 7
  
• In hardmode after defeating the Golem you will fight Betsy in wave 7 as well as all the previous mini-bosses (Dark Mage in wave 4,6 - Ogre in wave 5,6).

The event is kinda tricky since you can't really make an arena like you can with the other events and if you die the enemies will break the Crystal and it's over. But if you succeed you unlock several Sentry Summon weapons that may be used in the world.
The enemies will spawn anywhere from 60-120 tiles away depending on how large your arena is.

Queen Slime

A new optional boss in 1.4 once you hit Hardmode


Gelatinous Gem: When used it summons the boss while in the Hallow Biome.
To find it, travel into the underground Hallow where you can find Crystal Shards growing on Pearlstone and Pink Ice. Find and break the glowing purple gem that can spawn randomly to gain the item. They can grow in any direction.

Simply find another Gelatinous Gem to refight, there can be several present in the same map as seen in the picture below:
https://steamcommunity.com/sharedfiles/filedetails/?id=2140758827

The Mechanical Trio

Mechanized versions of previous normal mode bosses that unlock better equipment, and challenge.


After breaking at least 1 Demon /Crimson altar with the hammer in your hardmode world there is now a random chance (10% apparently) that one of the Mechanical bosses(The Twins, The Destroyer, Skeletron Prime) will spawn each night. A message will appear at the bottom of the screen to signal this to the player. A mechanical boss previously defeated will not randomly appear. (This regards to the world and not the character.)
Likewise there is a really low chance that any enemy in the game will drop the summon item for the boss before the boss has been defeated. Some players therefore postpone defeating the mechanical bosses to stock up on several of these.

Note that a world without Iron can use Lead instead. And that the summon items can be used in a world that haven't activated hardmode yet.

The Twins

"This is going to be a terrible night..."

Mechanical Eye:Crafted at a Mythril / Orichalcum Anvil using 3 Lenses, 5 Iron bars(15 ore) and 6 Souls of Light. Lenses are dropped by demon eyes during night and Souls of Light can be found from any type of monster in an Underground, Cavern or Underworld Hallowed biome. it is easy to spot an underground hallowed area with the glowing pink enemies and crystal shards growing on pearlstone. Only summoned during night, will despawn once morning comes.

The Destroyer

"You feel vibrations from below..."
Mechanical Worm:Crafted at a Mythril / Orichalcum Anvil using 6 Rotten Chunks / Vertebrae, 5 Iron bars(15 ore) and 6 Souls of Night. Rotten chunks are dropped by corruption enemies and Vertebrae by crimson enemies. Souls of Night can be found from any type of monster in an Underground, Cavern or Underworld Corruption or Crimson biome. it is easy to spot an underground Corruption area with the appearance of Clingers, Worldfeeders and the dark background. Underground Crimson areas are filled with the red stones and Ilchor spitters and Floaty Gross. Only summoned during night, will despawn once morning comes.

Skeletron Prime

"You feel the air getting colder"

Mechanical Skull:Crafted at a Mythril / Orichalcum Anvil using 30 bones, 5 Iron bars(15 ore) 3 Souls of Night and 3 Souls of Light. Bones are random drops by the enemies in the dungeon. Killing skeletons outside the dungeon gives no bones. The information about the Souls can be found above. Only summoned during night, will despawn once morning comes.

Plantera

"The jungle grows restless"



Once all 3 Mechanical bosses have been defeated a message will appear at the bottom of the screen to tell you that something is going on in the jungle. This means that an objects known as a "Plantera Bulb" has spawned in the jungle biome (and some other stuff). Breaking this object with a Pickaxe summons the boss. The bulb needs 2 tiles of flat ground to grow but some jungle objects (not grass) can obstruct possible spawning points.

Note that Plantera enrages when outside the underground Jungle. And that is not the same thing as the "layers" shown by the deph meter:
"Natural Underground Jungles begin below -60 feet in a small world, -90 feet in a medium world, and -120 feet in a large world." - Wiki

Some people have noted that plantera bulbs don't seem to spawn. This usually occurs during speedruns when a world is created, the Wall of flesh is defeated and then the Mecha Trio and nothing else. It is uncertain what causes this but some say that it worked after they defeated Queen bee.

Plantera Bulb farm:
There is no item that summons Plantera but breaking another plantera bulb will summon a new one. There is at least 1 in the world and new ones grow randomly on jungle grass. This means that you can place mud and put Jungle Grass Seeds on them to grow your own underground jungle and have them grow there. And while they will grow on any jungle grass block below 0, it is still adviced to place enough jungle blocks to create a jungle biome since Plantera will enrage when fought outside the jungle biome.
Sometimes the natural jungle is unusually bumpy and might prevent the bulbs from spawning. However, they will overwrite normal plats when they appear at least.
Killing more of the mechanical bosses seem to force another plantera bulb to spawn.(?)
http://steamcommunity.com/sharedfiles/filedetails/?id=499192072

Empress of Light

A new optional boss in 1.4 that becomes available post Plantera

You need to find and kill a rare spawning enemy called the Prismatic Lacewing which will create an overhead ray of light effect followed by the boss.
The boss has a special enrage mode during daytime where all attacks basically kill you in one hit, but if the boss is summoned and defeated in daytime alone it may drop a special item.

Prismatic Lacewing: They spawn in the overworld Hallow, apparently they only spawn during night after Plantera has been defeated (?).
To avoid the random nature of the fight because of this you can also capture them with bugnets despite being enemies. Then release it and kill it to summon the boss.
The Lifeform Analyser can detect them as well.

PS since they are enemies and not critters the Critter Handbook that prevents critter to die will not prevent your from accidently summoning the boss.

The Golem

The sleeping protector of the Lizhard Temple



Killing plantera grants the player a Temple Key, this temple key allows you to enter a large building inside the cavern layer of the jungle called "The Temple". There are several chests, traps and monsters inside the temple. The chests can contain and enemies can drop "Lizhard Power Cell".
At the final room of the temple there is an object called "Sun Altar" that you can right click to summon The Golem if you have a Lizhard Power cell in your inventory.
Note that even if you break into the temple using a pickaxe, The Golem will not be summoned until Plantera is defeated.

Sun Altar:The sun altar is in fact a furniture that can be picked up and moved from the temple if you have the Picksaw the Golem drops or better. The Golem has no enrage mode when outside the temple so it's nearly adviced to do(This was changed in 1.4, it now enrages outside the biome, just like Plantera.) However, you still need to gather Lizhard Power Cells to activate it. All the Lizhard enemies in the temple has a small chance to drop Lizhard Power Cells.

Duke Fishron

"Hey! Just the sacrifi- I mean competent fishing master that I've been looking for!"



As part of the fishing update of 2014 a new optional boss was added, this time in the Ocean Biome. However, summoning it is a bit more difficult.

To summon Duke Fishron you need to fish in the ocean biome using a Truffle Worm as bait in Hardmode.
To fish you need a Fishing Rod of any quality. The cheapest is made from wood. Simply use your fishing rod in the ocean with a Truffle Worm in your inventory, wait for the blubber to move, click the button to reel in and you will get a message stating that Duke Fishron has awoken. Fishron will enrage outside the ocean biome so if your ocean is Hallowed, Corrupted or crimsoned then Fishron will enrage (Not sure if this was fixed).

Truffle Worm:To hook a Duke Fishron you need to use Truffle Worms as bait. These are critters only found in hardmode in an Underground Mushroom biome or lower. Since they are critters this means that other enemies will kill them on touch and that your attacks will kill them as well.
What you need to do is catch it using a Bugnet. The Bugnet is sold by the merchant.
If you are carrying a Lifeform Analyser or any upgraded version of it then it will detect the Truffle Worm as a "Rare creature" when it is nearby. Other than that, I would suggest expanding a mushroom biome horizontaly to create more room for the truffle worm to spawn. And if you find a spot where you got one, chances are that a new one will spawn in the same location later.
Also, there are several Truffle Worm AFK farms that you can find on youtube.

Hardmode Events

Pirate Invasion:

The first hardmode invasion. Much like the Mechanical bosses you need to break at least 1 Demon /Crimson Altar to make these happen. They will happen randomly much like the Goblin Invasion at the start of the day. The ammount of pirates needed to kill is counted with the progression bar. A Mini boss called the Flying Dutchman will spawn during the event.

Pirate Map:Any monster present in the hardmode Ocean Biome has a chance to drop the Pirate Map. The item will summon a Pirate invasion at any time.

Solar Eclipse:

Much like a blood moon but this will last the entire Day. Unique enemies spawn during this event and has a chance of starting at the begining of each day. Note that the difficulty of the event increases as you progress in hardmode. Several new enemies is added once you defeat a mechanical boss and again when you defeat plantera.

Solar tablet:You can summon a Solar Eclipse at any time during the day by using a Solar Tablet. These can be found in chests in the Jungle Temple. You can also craft them using 8 Solar Tablet Fragments at a Mythril /Orichalcum Anvil. The fragments are randomly dropped by enemies in the Jungle Temple. So you will most likely get some as you farm for Lizhard Power Cells or the other way around.

Frost Legion:

The Frost Legion only happens when triggered by the player. This event can be summoned at any time. Clearing the event makes the Santa NPC move in during December (15-31).

Snow Globe: Although the event can be triggered at any time you can only get the Snow Globes from Presents. Presents are randomly dropped from any enemy during the Christmas Holiday period of December 15 to 31.
Added in 1.4 Completing the Frost Moon Event makes the following day count as "Christmas Holiday". The Presents contain random items and getting a Snow Globe can sometimes be a choir.
Note, snow globes can be used in a normal world as well, but will only drop in hardmode.

Lunar Bosses

Lunar Cultist

That boss that makes it feel like a PvP game.


Once you have defeated The Golem, 4 Cultists will appear at the entrance to the Dungeon where the Old Man previously stood. To summon the Lunar Cultist you merely need to kill the 4 cultists.

Just note that this will start a chain of events starting with the boss fight with the Lunar Cultist, if you win it is followed by an event that changes your world and can't be canceled and ends withthe final boss fight.

If you wish to refight the Lunar Cultist you merely have to return to the entrance of the Dungeon 1 day later as the cultists have respawned. Just note that the chain of events will begin once more.

Some players have reported a problem with the cultists not spawning at the entrance if they have placed furniture there or removed dungeon bricks from the "entrance".
More info on that here:
https://youtu.be/KkhO0kcgSkM

Moon Lord

"Your inpending doom approaches"



To summon the final boss of the game you need to complete the Lunar Event. The Lunar Event starts with you defeating the 4 cultists spawned at the entrance of the dungeon. Doing this summons the Lunar Cultist, defeating the Lunar Cultist begins the Lunar event where 4 pillars are spawned in the world and needs to be defeated. Once the 4th pillar is defeated some minutes will pass before the Moon Lord spawns at your location. It's hard to miss his awakening as the game goes silent, is tinted grey and some static is seen.

Celestial Sigil:If you want to refight the Moon Lord but don't want to go throught the Lunatic Cultist and the Lunar events you can craft a celestial Sigil. This is crafted at the Ancient Manipulator with 20 Solar Fragments, 20 Vortex Fragments, 20 Nebula Fragments and 20 Stardust Fragments. Completing the Lunar events usually gives you around 30 of each fragment. So each completed event is 1 free moon lord battle. Note that this item cannot be used unless The Golem has been defeated.

Endgame Events

Martian Madness:

After defeating the Golem there is a small chance that a Martian Probe will spawn at the edges of the map. Much like the Goblin Scout. To initiate the invasion you need to move close to the probe or attack it, but do not kill it. Upon detecting you it will flee and the martians will invade.
During the event a mini boss called the Martian Saucer will spawn and drop several unique items. If you kill it, nothing bad happens. It drops nothing and a new one will probably float on by in a minute or so.

There is no other way to summon this event besides finding a probe. Several players have noted that probes rarely spawn or not at all. There are some homemade fixes to these problems but they are hard to test. Some tips will be added here however.

• -The probes are rare creatures and can be spotted with a lifeform analyser.
  
• -Using battle potions and water candles increase spawn rates.
  
• -They are more common the further away from the middle you get, so being at the edges of the world near the ocean biome is a good bet.
  
• -They are also more likely to spawn at high altitude, such as sky or space.
  
• -Some people noticed that they reappeared after they defeated the golem again.
  
• -Killing the probe spawns no event but a new probe can reappear nearly instantly, however some say that you need to wait a whole moon cycle(same as sundial) for them to reappear once you have completed the event.
  
• -Some people claim that having the cultists appear on the screen at any point blocks the spawn of the probes so players should no go to the side where the dungeon is. (Eternisalt4pres reported that the martian probe appeared above the dungeon where the cultists were raving)

http://steamcommunity.com/sharedfiles/filedetails/?id=496800288
http://steamcommunity.com/sharedfiles/filedetails/?id=496800467

Pumpkin Moon:

The pumpkin moon event is a halloween themed event that is only triggered by the player. This event can only be summoned during night and lasts until morning where then it ends. Note that it is not limited to a certain date.
There are 2 mini bosses in the event with several useful items. The event consists of waves, each wave defeated increases drop chance for the monsters.
After clearing the event, the following day will have the seasonal hallowen spawns and items. (1.4)

Pumpkin Moon Medallion: To craft the item at a Mythril / Orichalcum Anvil you need 30 Pumpkin, 5 Ectoplasm and 10 Hallowed Bars. Pumpkins grow naturally during the halloween period of October - November but can also be grown using Pumpkin seeds sold by the Dryad. Ectoplasm are dropped by Dungeon Spirits that sometimes spawn when you defeat monsters in the Dungeon. However, you need to defeat Plantera first. Hallowed bars are dropped by each of the mechanical bosses (Twins, Destroyer, Skeletron Prime)

Frost Moon:

The Frost moon event is a christmas themed event that is only triggered by the player. This event can only be summoned during night and lasts until morning where then it ends. Note that it is not limited to a certain date.
There are 3 mini bosses in the event with several useful items. The event consists of waves, each wave defeated increases drop chance for the monsters.
After clearing the event, the following day will have the seasonal chirstmas spawns and items. (1.4)

Naughty Present:To craft the item at a Mythril / Orichalcum Anvil you need 20 Silk, 5 Ectoplasm and 5 Soul of Fright. Silk is created at a Loom using Cobweb as material. Ectoplasm is dropped by dungeon spirits after plantera and Soul of Fright is a drop from Skeletron Prime.

Lunar Events

This event is part of a chain where the player defeats the Lunar Cultist first, after the boss is dead the event starts and spawns 4 pillars in your world, 2 on each side of the middle. Each pillar changes the background of that area and replaces all spawns with new enemies. To destroy the pillars you need to defeat enough (around 150) unique enemies. Once the right number is reached, the pillar's health bar will be revealed and damage can be made.
Each pillar has unique enemies and each pillar drops Fragments that can be used to craft end game gear. Upon defeating the last pillar, the Moon Lord will spawn minutes later.

Unlocks from the Bosses and Events

These are the changes and new things that become available by completing the above events and bosse.

EoW/BoC
-Crafting materials to craft gear with Deamonite/Crimtane
-Meteorites can now randomly spawn
-Chance for random Goblin Invasions
-Tavernkeep NPC can spawn(for the Defenders Event)

Goblin Invasion
-Goblin Tinkerer NPC can spawn, your source for reforging items and buying the Tinker's table to combine Accessories

Queen Bee
-Witchdoctor NPC

Skeletron
-Access Dungeon (which leads to Shadow Keys)
-Clothier NPC
-Mechanic NPC can spawn in the Dungeon

Defender's Event
-Can use defender's Sentries outside event
-Defender's Medals for the Tavernkeep's items (the largest personal storage can be bought)

WoF
-Changes world into Hardmode
-Pwnhammer
-Class specific accessories

Pirate Invasion:
-Pirate NPC
-Credit card accessory

Mecha Trio:
-Killing first one unlocks:
- Solar Eclipses
- Defenders event upgrade
- Steampunker NPC (Clentaminator FTW)
-Killing all turns Underground Jungle into Hardmode version ( more Life Fruit, Chlorophyte)
and Plantera Bulbs can spawn)

Plantera
-Slows down Corruption/Crimson/Hallow Spread
-Access to Jungle Temple
-Dungeon upgrades
-Can open Biome Chests in Dungeon with Biome Keys
-Cyborg NPC
-EoL optional boss
-More Solar Eclipse enemies

Golem:
-Lunatic Cultist at the top of Dungeon
-Defenders event upgrade
-Martian Madness event

[SuspiciousLookingDog]

an actually usefull guide :) :)

[AgarthanTerrorist]

This one time I was fishing for a quest and forgot I had truffle worms as bait

[Blazemico]

i live in the dungeon and i saw 4

[Blazemico]

I DID OLD ONE`S ARMY IN NORMAL

[SUPER-TANK]

Better WOF Tooltip: The Ancient Spirits of Light and Dark has been released
(I Know it's post WOF but it's sounds cool)
Also, Pirate Invasion will only progress and spawn enemies if one housed NPC is nearby. I used this and took the fight in the ocean (celestial shell), which is a good way to farm Pirate Maps.
End-Game items, especially Last Prism, can be used to farm maps (Because of Pirate's Rare drops). Just reach wave 15 quickly, then go to the ocean. All the mourning woods will arrive and since they respawn quickly and easy to kill, got 2 maps per event per average.

[SUPER-TANK]

Due to a bug, if the player chases the Martian Probe all the way up, it will vanish and the event does not occur (Luckily I have all 9 Drops from the saucer, I'm just curious)

[Alchemist]

yeah but what is an arena for an eternia crystal like wait what help me what is an arena for old ones army what do i have to build also plz how big can it be and also it it sucide to do this with like demonite head, scalemail, no boots and like a platinum sword demon scyphe and a musket and flame turret thing
help

[Roadbuster_DoM]

wait theres an achievement for the old ones army?

[Exotic.]

the goblin army is likely caused by goblin scout, found by the edges of your map

[fvtvrismo_glavoverkh]

The destroyer just fled when I took a greater HP potion ;-;