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
Donate each sin to the museum in turn
Tell the Stockade Guard that the conspiracy is real
Guess wrong on each of the riddler's riddles
Refuse to think what you must, five times
Leave the Narthex without an answer
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
Try to touch myself with extra sins after receiving the medal
Use the grail
Find all 100 obols
Complete the Museum labyrinth
Reach all six endings
Complete the Order labyrinth
Free all four demons
Enter all five places of power
Collect all five treasures
Join all five orders
Uncover all five great lies
Expose all five cabbages
Switch parties five times
Learn all five secret words
Learn all sins
Kick skull spiders 23 times