This WoW set features the full Heroes’ Plagueheart transmog, which is the warlock tier 7 (10-man) raid set. Plagueheart is obtained by looting Wrath of the Lich King bosses and purchasing some set pieces straight from the vendor with gold. Dropped tokens can be traded for set pieces via Dalaran tier vendors.
How to Get Warlock Tier 7? By purchasing from Dalaran vendors and slaying Naxxramas bosses.
How Many Set Pieces?  Helmet, Shoulders, Chest, Hands, and Pants
Warlock Tier 7 Transmog: [10-Man Drop Locations]
Head: Helm of the Lost Conqueror [Kel’Thuzad] in Naxxramas
Chest: Chestguard of the Lost Conqueror [Purchase from Vendor] in Dalaran.
Hands: Gloves of the Lost Conqueror [Purchase from Vendor] in Dalaran.
Legs: Leggings of the Lost Conqueror [Thaddius – Gluth] in Naxxramas.
Chest/Hand Vendor (Alliance): Arcanist Ivrenne [37,55] in Dalaran.
Chest/Hand Vendor (Horde): Magistrix Lambriesse [65,23] in Dalaran.
Redeem Token Vendor (Both): Paldesse [43,49] in Dalaran.