Cloak of Rot

Source
This item dropped from Lord Malathrom, a level 31 rare mob ghoul in the Dawning Woods Catacombs in Duskwood prior to Cataclysm. It is now available again via Crate of Salvage and Big Crate of Salvage.

Trivia
Cloak of Rot is among the very few items from classic World of Warcraft that give a negative bonus to a stat.