Yggdrasil
The Eldritch Forest God, Caesin Bapri
Difficulty: Pyrian Endgame

Yggdrasil was introduced in Pyria patch 20.0

It is one of the mod's few “tameable” bosses, requiring defeat in order to obtain. However, Yggdrasil is one of the toughest bosses to date, being extremely powerful and summoning in extremely dangerous minions. Let's recap a few basic information bullets about Yggdrasil:

-Spawns in Fertile Chamber, Jungle, Redwoods
-Lasts 1 hour in wild before vanishing
-Weaknesses to Fire, Explosions, Minorly weak to Electricity.
-Summons in Dimorphodons and Ironwood Ents
-spawns at 2x the maximum level cap of your server.(edited)
Yggdrasil is a true monster in every right, even if it is benevolent and passive. Only challenge one with a FIRM gameplan in mind. To help you gather one, here's the list of creatures, both in the mod and vanilla, I see as viable for a Yggdrasil fight, and I'll explain why in a bit:
-Drake
-Smok
-Escadora
-Glacier Golem
-Ironwood Ent
-Titanosaurus (Yes, titan)
-Giganotosaurus (be weary until 21.0)
-Alpha Infernal Tyrannosaurus
-Alpha Fulgur Carnotaurus
-Dark Mana Reaper Empress
-Dark Mana Reaper Drone
-Balrog
-Incinerox
-Eldritch Hailfire Wyvern
-Yutyrannus
-Daeodon
-Miralis Yutyrannus
-Infernal Basilisk
-Hydra
-Dark Mana Seeker
-Fire Wyvern
-Divine Fatewalker Eminence
-Lightning Wyvern

Alright, with that in mind, let's go over what the gameplan i'd recommend, via my own experience and being mod creator.
Your first step, in any right of course, is FINDING a Yggdrasil. If you're on a server I wouldn't recommend going after a Ygg that has been on since before you got on, in case of time. Yggdrasil can be a lengthy fight, so it's good to have all the time you can. Currently the dev kit is giving me a headache with timers, and although on servers it may SEEM like a Ygg's timer is stuck at 1 hour, it's still going down slowly. Sentinel is a good example of this weird problem, as are Dark Mana Reapers. That aside, if there's a Yggdrasil on since before you got on, it would be wise to ask people if they knew how long it's been on (if it spawned while they were on), and then determine how viable targeting said Yggdrasil truly is. It's best to always be at the ready and have your “Yggdrasil fighting army” ready at all times on standby. As soon as a Yggdrasil spawns, immediately send out scouts to determine it's location. On maps that have extensive jungles or redwoods, like The Island, The Center, and Ragnarok, finding a Ygg can be equivalent to finding a needle in a haystack, so it's best to to locate him as quickly as possible. If you have Classic Fliers, imprinted wyverns are excellent for quickly flying about to try and determine location, if you don't have a S+ tek transmitter (though with the update times on the transmitter, the Ygg may not register on it immediately). Smok, Drake, and imprinted Eldritch and Rok are also great scouters.
Mobilize immediately upon discovering your Yggdrasil. However, take into consideration location. It is best to fight Yggdrasil on as flat and unobscured a space as possible and avoid cliffs. Yggdrasil are not easy to kite in the slightest, so it is best to simply pass one up if the battlefield is not large enough. Now here's where things get tricky. We follow a few base rules here:

-No mounted player approaches Yggdrasil close enough for melee combat
-Keep tabs on Dimorphodons at all times.
-Yggdrasil buffs his dimorphodon and ent minions.
Yggdrasil has a deadly combo of stun-roaring, followed by Dimorphodon swarms. The swarms alone will tear a survivor apart in seconds and the mount soon after. That's where the majority of the mounts come in, and you may even want to forsake Daeodon in this situation to focus on other things. Ents will be a nuisance and can sure do damage very quickly, but they can easily be avoided or terminated. Your main threat is dismounting and Dimorphodons. This is where your creatures come into play.
Your main physical fighters should be tanky heavy-hitters. In the list above, this includes Drake, Balrog, Glacier Golem, Divine Fatewalker Eminence, Escadora, Hydra, and Alpha Infernal Tyrannosaurus, as well as Ironwood Ents and Giganotosaurus. Giga is especially VERY good due to his massive AOE per bite, but beware using Gigas until pyria patch 21.0 when the Serene Mind potion is implemented (one of the effects of this potion is temporarily preventing a giganotosaurus from becoming enraged). These heavy hitters have plenty of health to take Yggdrasil's direct punishment, as well as the swarms of dimorphodon. Large AoE hitters like Glacier Golem, Ent, and Balrog- especially Hydra- are great for this. Using their large aoe, if lucky, they can terminate the Dimorphodons quickly and leave simply the Ents remaining. Using tamed Ironwoods as well is considerable to to the fact they too will be buffed by the ENEMY Yggdrasil, becoming stronger to help assist in your battle.
Titanosaurus is also valid due to heavy damage and AoE, though the practicality is debatable.

Dark Mana Reaper Empress is also capable of standing her ground against Yggdrasil, but he SERIOUSLY gives her a run for her money. Once again the dimorph swarms will be a problem here.

The rest of the creatures should be self-explanatory, given their roles, especially Daeodon and Yutyrannus. Alpha Fulgur Carno might seem random, but remember: his presence alone buffs nearby Yutyrannus and Miralis Yuty. Should you be using Miralis Yutyranus, consider having a mounted Alpha Fulgur carno for support, or actual fighters if you have a mate-boosted pair with heavy armored saddles.

The rest of the fighters are your ranged bombers. Incinerox, Dark Mana Reaper Drones, and Infernal Basilisk are EXCELLENT mounted ranged support: especially in no friendly fire servers. Their explosions both do increased damage to ents and yggdrasil, but the explosion AOE upon Yggdrasil itself can help kill any Dimorphodons that aren't destroyed immediately. Fatewalker ranks as a tank due to her heavy attacks and health, but most crucially:
her explosion spamming

not also is this explosion spamming extremely powerful, but it's completely devastating to a massive animal like Yggdrasil, and the explosions could also take out nearby ents or dimorphodons- always a bonus.

Smok, Lightning Wyvern, Fire Wyvern, and Eldritch Hailfire Wyvern in this scenario are your hit-and-run air fighters. Smok and the Wyvern's breath attacks do increased damage to the Ygg and its Ent minions, whereas Eldritch's Bazel Crystal bombs do increased damage to them, have a considerable AOE, and are devastating on large targets. If your ground tanks deal enough damage and can control the dimorphodons easily, you can even hover above the ygg a distance up without needing to worry about them flying up to you.

Do note: Yggdrasil is a LONG fight due to how tanky he is. Expect things to die. I've estimated that a 360 Yggdrasil could have at least 600,000+ health (scanova has confirmed 300 Yggdrasil can have nearly 1,000,000), from seeing it take so much abuse from an imprinted high-level drake

However, do note: when the beast is finally slain, you can use the Immortal Seed you obtain to grow the baby Yggdrasil back. Although tamed Yggdrasil don't have as much damage or the same stats as their wild boss counterparts: you can imprint on them and make up for that.

Yggdrasil is a powerful creature that is hard to obtain: but worth it when you do.

A final closing note:

When on low health, Yggdrasil will begin fleeing for a while, until it eventually returns for more combat. Use the opportunity to deal serious damage.

Personal experience shows a very high level and full imprinted Poison Wyvern can level Arch Demons, Eminences, and Ogres. And with a Water Wyvern can level a Colossus with trouble.

Nice!

Going to make all bosses immune to em now.

Please no, I beg

sorry dude, but im all about balance and avoiding exploits.

as far as i can see, those are exploits.

im sorry mate, but it's already been done and is cooking now.

Sad times, but nah, I understand. What I would do is land on rocks and shoot at them and they had no way of hitting, my wyverns are just nasty strong so the colossus thing nearly kills me everytime. (The "please no, i beg" was a joke so you don't need to apologize, still love the mod the same man)

Lol

Anywho guys i'll probably be covering some other bosses within the day, i plan on having Ogre, Diorex, and Insidious eminence soon. Ogre will likely be later today.

so once youve defeated this thing, how the heck do you "grow" a new one with its seed? Still new to this mod and trying to figure out this stuff

Just use the item in an open space

Hey, do you think you can make some of the bosses undrownable? I've been able to lead the Empress and the Insidious into the ocean and have them drown without a fight, leaving their lifeless body able to loot. It feels like a cheat when they are such strong monsters.

Yeah

I'll make all of em immune to it

Neat! Great work on the mod so far :)

Can any pyria mobs be used in the Ragnarok Arena Boss fight? The one where you're teleported to it and you must fight the dragon and manticore at the same time. Would any be better to bring than the typical Rexs, yutyrannus, and pigs?

Dagon, but dagon will take the majority of the teleporter space. Same with brute wyverns.

Divines and their Allosaurs are also really good too!

Miralis Yutyrannus, Orthros and Balrog are also really good!

what do you feed a tamed yggdrasil?