Twilight Trappings

Twilight Trappings was a quest set required for summonings at Wind stones in Silithus prior to Cataclysm. Equipping all three applied the aura spell "Twilight Cultist Disguise".

While the item set became useless with the removal of the related Silithus quests, the set bonus remained in the game until Patch 7.1.5.

Source
Each piece drops from Twilight's Hammer cultists in Silithus.