Who It Suits
Leather is the armor type Demon Hunter, Druid, Monk and Rogue wear. With Agility or Intellect, Nightstalker's Chestguard suits Assassination, Outlaw and Subtlety Rogue; Brewmaster, Mistweaver and Windwalker Monk; Balance, Feral, Guardian and Restoration Druid; Havoc, Vengeance and Devourer Demon Hunter.
Transmog Sets
Nightstalker's Chestguard is part of the transmog set Guise of the Nightstalker (Rogue, Monk, Druid and Demon Hunter only). The other 16 pieces:
Built from the World of Warcraft client build files and recorded Auction House snapshots.