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