How to Unlock The Secret Boss
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 5 of the following cards in your deck to spawn the boss:
You will also need the Oblivion card for this to work.
Spawning the boss
Once you have 5 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 all 5 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!