Here is the full list of all 39 Cathedral achievements.
Kill Ardur
Find all map pieces
Destroy the Conduit
Find and activate all portals
Return 80 books
Acquire all Ouroboros amulets
Slay all gargoyles
Kill the blood knot
Find all hearts
Kill Oloc
Return 60 books
Kill Nidlong
Buy the Onyx armor
Return 40 books
Kill Thalia
Buy the Tourmaline armor
Return 20 books
Kill Salemacke
Kill 150 skulls (of any kind)
Kill the Necromancer
Kill 100 skulls (of any kind)
Kill 50 skulls (of any kind)
Have more than 10 000 gold stored in the bank
Have more than 25 000 gold stored in the bank
Have more than 50 000 gold stored in the bank
Buy the Emerald armor
Kill 150 blobs (of any kind)
Kill Augr
Kill 100 blobs (of any kind)
Kill 50 blobs (of any kind)
Destroy an electric orb in Necromancer's den using the lantern
Die one hundred times
Acquire all health bottles
Find all ammo upgrades for the Magnus Apparatus
Find all ammo upgrades for the Flintlock
Return 100 books
Find all charms
Complete the game without upgrading the armor
Find all armor augmentations