This quest is not available in game.
Enter Blackrock Depths and track down Lord Incendius. Slay him and return any source of information you may find to Thunderheart.
I can taste the foulness in the air that surrounds you, <name>. There is another, hidden in the depths of Blackrock, who does control this foulness.
The wind and earth cry his name: Lord Incendius... but someone... something... commands this being. He is merely an emissary.
Find him and discover where his master hides. Return to me when you have collected this information.