http://www.uesp.net/wiki/Skyrim:The_Legend_of_Red_Eagle
One night, under a cloud-choked sky, the men of the Red Eagle warmed themselves over damp fires of smoldering moss. A huddled, shambling figure came to them, cloaked in rags, face cowled. Though his men mocked and cast stones at the stranger, Faolan sensed something, and beckoned. The cowl was thrown back in the dim light, and she revealed herself to be one the ancient and venerable Hagravens. She offered power, for a price, and a pact was made.
Thus was brokered to the witch: his heart, his will, his humanity. From that day forth, his was a spirit of vengeance, pitiless and beyond remorse. The rebels grew in strength and numbers, and none could stand against them. Faolan's eyes burned coldly in those days, black opals reflecting a mind not entirely his own. Two years passed, and the foreigners were all but driven from the Reach.