Travel to Blackrock Spire and find Goraluk Anvilcrack. Slay him and then use the Blood Stained Pike upon his corpse. After his soul has been siphoned, the pike will be Soul Stained. You must also find the Unforged Rune Covered Breastplate. Return both the Soul Stained Pike and the Unforged Rune Covered Breastplate to Lorax in Winterspring.
Lorax makes deals, he does not accept them from insignificant mortals.
A deal I shall make with you, however... take this pike, it is imbued with powerful Fel energies. When you find Goraluk Anvilcrack, slay him and drive this pike through his dying heart. His soul he bartered, his soul I shall have. Return the pike and return the breastplate that the thief stole and I shall teach you what it is that he was to learn.