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