Here is the full list of all 18 The Unholy Society achievements.
Finish Act I.
Find Comic book V.
Find all comic books.
Defeat Belial.
Find Comic book IV.
Defeat Werewolf.
Defeat Aunt and Uncle.
Find Comic book III.
Defeat Cultist.
Defeat Conductor.
Call Pope for the first time.
Defeat Eduard.
Find Comic book II.
Defeat Sadako in battle.
Give the VHS to Sadako.
Find Comic book I.
Defeat Cat.
Defeat Lilith.