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