It's located in a fairly large area, signified by a T shaped walkway, wooden walkway, a slight cliff, and 4 Captive Guards. The area swarms with Fallen Peons, Fallen Masters, and includes a Fallen Conjurer or two.
The event doesn't officially start until the player frees one of the Captive Guards, so one can clear the area before freeing the Guards. Freed guard will either thank you or say he must flee, which both answers will have them run away. After freeing all 4, a unique monster, Bramok the Overlord, will spawn at the foot of the wooden walkway. Defeating it will complete the event and rewards gold, experience, and the challenge "Prisoners of Kamyr".