Plagueheart Raiment

Plagueheart Raiment is the Tier 3 set for Warlocks.

Source
Most Tier 3 items can be acquired through quests given by Mataus the Wrathcaster, who can be found in Light's Hope Chapel in the Eastern Plaguelands. Before the quests become active, must be completed.

The quests require several crafting materials, s (which drop from Trash Mobs in Naxxramas) as well as a token from a boss in Naxxramas

The only exception is the, which drops directly from Kel'Thuzad in Naxxramas