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