How to unlock the Secret Boss

This is a Spoiler, so read at your own risk!!!

Unlock Conditions

To unlock the secret boss, you must have completed Act 2 (beaten the Priestess). Once you have completed Act 2, you can spawn the boss on any Act.

You will need at least 6 of the following cards in your deck to spawn the boss:

• Vampire Mansion
  
• Chrono Crystals
  
• Battlefield
  
• Blood Grove
  
• Beacon
  
• Bookery
  
• Temporal Beacon
  
• Roadside Lantern

You will also need the Oblivion card for this to work.

Spawning the boss

Once you have 6 of the listed cards and Oblivion in your deck, start a loop and look for a U-shaped bend in the loop. This will be where you start placing tiles. Your goal is to create a road tile that is overlapped by at least 6 tile effects, and if you have done it correctly you will see blue sparks on the tile. Once you see these sparks, place an Oblivion card on that tile and congrats, you have spawned the boss!

(there are 6 overlapping effects here, but you only need 5 to spawn the boss)

The boss is really hard, but you get to listen to some dope boss fight music straight from another game you might recognize!

Credit

Full credit goes to the Loop Hero subreddit for providing almost all of the information in this guide! The Loop Hero discord and subreddit have a lot of tips and tricks to figure out everything this game has to offer, so check it out if you want help!

[Yggdrasil Burnes]

There's also Armac, who is spawned by a perk.
Armac is basically a giant angry face perk that only appears in the level up screen if you "Breakfast" through all of the actual skills available. You will never see him in a normal game unless you have a single game run for months straight. The EXP gain is simply not possible.
Cheating in EXP will let you get all the perks available to you, but when the last two perks are cycled in, Armac will replace the third one. He will inevitably be the only perk left to pick up, and doing so will spawn him on the map.

Every one of his attacks removes a perk, and you basically do nothing to him.

Realy big funni.

[AranRaich]

Thank you, its a great guide. Since beacons and vampire mansions don't stack, I would have never bothered to try this on my own, and would have missed it. Also there is another secret boss called Frog king (like Mortimaar said): He has 2 unique dialogs (First time meeting, Second time meeting) and gives no rewards after defeat. You can spawn multiple of them by using oblivion on swamps and then filling the board with all swamps again.

[Mortimaar]

There are multiple secret bosses. So far there are 2 i know. This one and the Frog king. Frog king: all road tiles are covered in swamps. When you place the last swamp the Frog king will spawn on the swamp tile right before the camp (end of the loop). I recommend the thief for that. Additional cards: garrison, outpost (covering every inch), spider cocoon (for card draw), oblivion, aresenal and the usual for thief (thickets, meadow, river, suburb). skillwise: everything that helps to survive that long until the road is covered in swamps. I did it on stage 1 and made about 36 loops until i drew and placed all swamp cards.

[notwoodenshoes]

Yeah, I think there's still some info missing on the exact spawning mechanics, I just posted what was known at the time, so if some things are wrong I apologize!

[Mav]

This is incorrect: "you must have completed Act 2 (beaten the Priestess)" - I had't beaten chapter 2 yet and spawned the sphere in chapter 1 on loop 11, one full loop after doing the boss on loop 9. You also don't really need a U-shaped road specifically but just need one road tile to be covered by 6 AOE ranged effects. I suspect you may be able to double up on temporal beacons and possibly lantern/beacons as well, in my screenshot on the reddit post I can't see any other 6th effect where the sphere spawned.

[Sperbs]

I just found out that the regular bookery and the abandoned bookery count as TWO different effects for the purpose of triggering this. And as Interpretive Twerking said, you definitely need 6 effects, not 5!

[AranRaich]

Thanks for the guide. Is there an achievement or something for beating the bosses multiple time? So far I've gotten a cutscene only. Also, using Warrior + Arsenal-card and Shoe Nails (camp item) seems like a good strategy, worked for me .

[Gott von Mung]

I just tested this and the blue sparks only appeared after placing 6 items in the U-shaped part of the loop. Cards used were Chrono Crystal, Battlefield, Vampire Mansion, Road Lantern, Beacon and Temporal Beacon. I tried multiple instances of 5-card influence with no results.

[CrouchingTiger]

sorry but i been trying to figure out what is the bottom right hand side tile. is that a lighthouse?