The Encroaching Darkness is a divination card. A set of eight can be exchanged for a random corrupted unique map. This is limited to unique maps that can drop as loot, which excludes the following: The Beachhead (created with Harbinger's Orbs), The Perandus Manor (sold by Cadiro Perandus), The Vinktar Square (created with a vendor recipe) and Untainted Paradise (does not exist in item form).
Drop restrictions:
Ashen Wood Map
Arena Map
Arsenal Map
Barrows Map
Bazaar Map
Bog Map
Catacombs Map
Chateau Map
Courtyard Map
Coves Map
Crematorium Map
Mud Geyser Map
Museum Map
Promenade Map
Racecourse Map
Reef Map
Scriptorium Map
Shore Map
Spider Forest Map
Strand Map
Terrace Map
Thicket Map
Torture Chamber Map
Underground River Map
Underground Sea Map
Vault Map