Azzinoth

Azzinoth was a powerful doomguard commander of the Burning Legion slain by Illidan Stormrage during the War of the Ancients, 10,000 years ago. He was the original wielder of Illidan's warglaives, which the demon hunter claimed after slaying his foe.

Speculation
Demon hunters ingest the heart and blood of a demon to bond its spirit with theirs, granting them their demonic powers. Illidan may have done this with Azzinoth. The existence of the Soul Fragment of Azzinoth could support this.