Here is the full list of all 27 Theatre of Doom achievements.
Play a custom solo game
Play a custom versus game
You have just fallen into the void
Died on the first level
Buy all the upgrades
Complete a stage without taking any damage
Reach the first stage of the Machinamentum
Kill a demon puppet
Kill a doomed demon puppet
You have just wasted some Ectoplasm
Play a custom co-op game
Kill a doomed demon puppet while in Ecto-Rage
Killed by the other player
Fully upgrade the power triangle
Complete the game on Normal
Complete the game on Hard
Complete the game on Insane
Reach the exit teleport in less than 1 second
Reach the first stage of the Urbs
Reach the first stage of the Stagnum
Reach the first stage of the Glacies
Reach the first stage of the Arx
Reach the first stage of the Ignis
Defeat Lolligo Furiosus
Defeat Arbor Malus
Defeat Gigans Extinctor