15 transmog appearances in Naxxramas.
ArmageddonTwo-Hand
Broken PromiseOne-Hand
Charmed CiergeTwo-Hand
Claymore of Ancient PowerTwo-Hand
DamnationTwo-Hand
Final VoyageRanged
Gloves of Peaceful DeathLeather Hands
Gown of BlaumeuxCloth Chest
Helm of the GraveMail Head
Leggings of Voracious ShadowsMail Legs
Mantle of the CorruptedCloth Shoulder
Pauldrons of HavocMail Shoulder
Thane's Tainted GreathelmPlate Head
Urn of Lost MemoriesHeld In Off-hand
Zeliek's GauntletsPlate HandsSign in with Battle.net to see which of these appearances and collectibles you already have, and which of your characters can still loot The Four Horsemen this reset.