This quest is not available in game.
Retrieve an Obsidian Power Source and bring it to Tabetha in Dustwallow Marsh.
Now that the demon has been removed, we must gain a power source. Unfortunately, the best power sources are usually the oldest and the hardest to get. It's neat how fate likes to take a bite out of you sometimes, isn't it?
So you're going to need to go to Uldaman and defeat the obsidian sentinel there. He's big, he's tough, and his power source packs a punch!
I wish you the best of luck, <name>. And if you don't get lucky then, well... it was nice knowing you.