This WoW set features the full Valorous Frostfire transmog, which is the mage tier 7.5 (25-man) raid set. Frostfire 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 Mage Tier 7.5? By purchasing from vendors and slaying Naxxramas/Obsidian Sanctum bosses.
How Many Set Pieces?  Helmet, Shoulders, Chest, Hands, and Pants
Mage Tier 7.5 Transmog: [25-Man Drop Locations]
Head: Helm of the Lost Vanquisher [Kel’Thuzad] in Naxxramas
Chest: Chestguard of the Lost Vanquisher [Gluth] in Naxxramas
Hands: Gloves of the Lost Vanquisher [Sartharion] in The Obsidian Sanctum
Legs: Leggings of the Lost Vanquisher [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.