A Strange Key (Item)

"The key is marked with many strange runes and depraved symbols characteristic of the cult of Ch'thon."

This is a quest item used in the A Strange Key quest, and unlocks the Depraved Sanctuary.

The key can be obtained from Direni (if you let him live) in the A Cultist in the Midst quest.

It also has a chance to drop from Cultists in the Cultist's Lair or Burrwitch Outskirts: Zealots and Priests, with a 6% and 16% chance respectively to drop the key.