This WoW set features the full Valorous Plagueheart transmog, which is the warlock tier 7.5 (25-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 Lock Tier 7.5? By purchasing from vendors and slaying Naxxramas/Obsidian Sanctum bosses.
How Many Set Pieces?  Helmet, Shoulders, Chest, Hands, and Pants
Warlock Tier 7.5 Transmog: [25-Man Drop Locations]
Head: Crown of the Lost Conqueror [Kel’Thuzad] in Naxxramas
Chest: Breastplate of the Lost Conqueror [Gluth] in Naxxramas
Hands: Gauntlets of the Lost Conqueror [Sartharion] in The Obsidian Sanctum
Legs: Legplates of the Lost Conqueror [Purchase from Vendor] in Dalaran.
Shoulder/Leg Vendor (Alliance): Arcanist Ivrenne [37,55] in Dalaran.
Shoulder/Leg Vendor (Horde): Magistrix Lambriesse [65,23] in Dalaran.
Redeem Token Vendor (Both): Paldesse [43,49] in Dalaran.