This WoW set features the full Plagueheart Raiment transmog, which is the warlock tier 3 raid set. Plagueheart was orginally added in vanilla patch 1.11.0, then removed with the Naxxaramas revamp during Wrath of the Lich King, and finally made available once again in Mists of Pandaria via the Black Market Auction House.
How to get warlock tier 3? By bidding for the pieces on the BMAH.
How many set pieces?  Helmet, Shoulders, Chest, Bracer, Hands, Belt, Pants, and Feet.
Warlock Tier 3 Transmog:
Head: Plagueheart Circlet by purchasing the helmet from the BMAH.
Chest: Plagueheart Robe by purchasing the chest from the BMAH.
Wrist: Plagueheart Bindings by purchasing the bracers from the BMAH.
Hands: Plagueheart Gloves by purchasing the gloves from the BMAH.
Waist: Plagueheart Belt by purchasing the belt from the BMAH.
Legs: Plagueheart Leggings by purchasing the pants from the BMAH.
Feet: Plagueheart Sandals by purchasing the boots from the BMAH.