This quest is not available in game.
Plant the Stonemaul Banner near the entrance to Onyxia's Lair, wait for Onyxia's response, then return to Brogg at Mudsprocket.
Brogg relishes the thought of taking his revenge on the black dragonkin of the marsh. <name>, you have been very helpful in preparing for this moment. Will you help?
Take Brogg's clan banner and travel east until you see a cave like a dragon's mouth. At the entrance to that cave, plant Stonemaul banner and issue Brogg's challenge to Onyxia.
Brogg and the Stonemaul will have their lands back!