Here is the full list of all 35 Agony achievements.
Finish 4 level as a succubus
succubus ending
Finish 3 level as a succubus
Finish 2 level as a succubus
Kill a chort as a succubus
Finish 1 level as a succubus
Use a finisher on onoskelis as a succubus
Normal ending
Finish 4 level
Finish 3 level
Possess an ifrit
Kill an enormous chort
Possess a chort
Finish 2 level
Possess onoskelis
Finish 1 level
Nimrod ending
Good and Evil (Unlock Evil and Angel ending)
Baphomet ending
Find all secret rooms
Find 2 red chambers
Find 2 gold chambers
Max out every skill
Unlock all letters and notes
Unlock all paintings
Unlock all comic pages
Unlock all characters in 3d viewer
Unlock madman ending
Unlock everything in gallery
Furvive a level in Agony Mode
Find a peanut in the frozen caves
Try to push an onoskelis to the pit
Push and kill an onoskelis
Find a secret room
Survive Baphomet bossfight