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The Murder Pens is located deep inside Razorfen Downs, and is where the dungeon's three friendly npcs, Belnistrasz, Henry Stern and Sah'rhee are being held prisoner. As the name of the area suggests, the three humanoids are not going to remain alive for very long.
The Murder Pens lies beyond the Caller's Chamber, and just across a small bridge from the Bone Pile. The holding pens are breakable, and releasing the prisoners allows a party to obtain the quest from Belnistrasz, and learn the recipes for Mighty Troll's Blood Potion (alchemy) and (cooking) from Henry Stern.