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