Key to Searing Gorge

Source
This item is a quest reward from a quest chain that starts with The Horn of the Beast and ends with At Last!.

To get the first quest in the chain, you must first obtain Margol's Horn by killing Margol the Rager in Searing Gorge.

Note
Seeing how the gate to Searing Gorge is now open to all. Having this after 4.0.3a now seems pointless. Also quests seems to been removed as well.