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