The Slave Pens is one of the dungeons in Burning Crusade Classic. It can be accessed from level 59. The entrance is located in the western part of the Coilfang Reservoir in Zangarmarsh.
Location
Note that in order to access it in Heroic, you must be level 70 and possess the Reservoir Key, which is available to players who have reached the Revered reputation level with the Cenarion Expedition.