Loatheb was added in the Curse of Naxxramas Adventure. He is available from Wing 2 - The Plague Quarter which opened on July 29th, 2014, and is obtained after defeating the adventure boss Loatheb.
Adventure Guide
Need help conquering the Curse of Naxxramas Adventure to obtain Loatheb? Head on over to our Curse of Naxxramas Adventure Guide!
So much for Miracle Rogue xD