Here is the full list of all 26 Ossuary achievements.
Use a new sin on a new person, 555 times
Donate each sin to the museum in turn
Tell the Stockade Guard that the conspiracy is real
Tell the District 5 rep that they are disliked five times
Receive an answer at the Narthex
Refuse to think what you must, five times
Leave the Narthex without an answer
Try to touch myself with extra sins after receiving the medal
Speak to, and use all sins on, all knights in formation
Use the grail
Find all 100 obols
Complete the Order labyrinth
Free all four demons
Learn of the headless hydra
Reach all six endings
Learn all five secret words
Guess wrong on each of the riddler's riddles
Complete the Museum labyrinth
Uncover all five great lies
Collect all five treasures
Enter all five places of power
Join all five orders
Expose all five cabbages
Learn all sins
Kick skull spiders 23 times
Switch parties five times