From WoWWiki

The Forlorn Cavern is a long, winding road built around the City's underground lake. It's naturally dark and secluded nature makes it perfect for lots of shady deals. Warlock and Rogue trainers can be found here, as well as vendors catering to those classes.

To get to the Forlorn Cavern, you can head east from the Mystic Ward or west from the Hall of Explorers.

Subzones of Ironforge
The Commons • The Gates of Ironforge • The Great Forge • The Forlorn Cavern • Hall of Arms • Hall of Explorers • Hall of Mysteries • Hall of Thanes • The High Seat • The Library • The Military Ward • The Mystic Ward • Tinker Town • Vault of Ironforge