Here is the full list of all 39 Cathedral achievements.
Kill Ardur
Find all armor augmentations
Find all map pieces
Find all ammo upgrades for the Flintlock
Find all ammo upgrades for the Magnus Apparatus
Destroy the Conduit
Kill the blood knot
Find and activate all portals
Kill Oloc
Return 80 books
Kill Nidlong
Find all hearts
Find all charms
Buy the Onyx armor
Acquire all Ouroboros amulets
Slay all gargoyles
Return 60 books
Kill Thalia
Buy the Tourmaline armor
Return 40 books
Have more than 10 000 gold stored in the bank
Kill Salemacke
Kill the Necromancer
Kill 150 skulls (of any kind)
Kill 100 skulls (of any kind)
Kill 50 skulls (of any kind)
Buy the Emerald armor
Return 20 books
Die one hundred times
Kill 150 blobs (of any kind)
Kill 100 blobs (of any kind)
Kill Augr
Kill 50 blobs (of any kind)
Destroy an electric orb in Necromancer's den using the lantern
Acquire all health bottles
Return 100 books
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