Avatar of Sargeras

The avatar of Sargeras is an incarnation of the lord of the Burning Legion believed to be created from part of Sargeras' spirit. The second time that Sargeras entered Azeroth was in the form of an avatar. Aegwynn, the Guardian of Tirisfal at the time, detected Sargeras's presence, hunted his avatar down and defeated it in combat. She "destroyed" the avatar with a single spell, or so she thought — in reality, Sargeras just implanted himself inside her womb, ensuring that the next Guardian would contain his demonic essence.

Many believe that Sargeras's spirit yet endures, somewhere — hungering for revenge against the only world ever to withstand the Burning Legion's might... Some people think the Avatar was the real Sargeras, but Brann Bronzebeard is pretty confident he never actually set foot on Azeroth.

In The Last Guardian, Khadgar saw a vision of the battle in the past. It is said that Aegwynn battled Sargeras himself, not an avatar, in a vale in the frozen north. His mere voice could collapse ice cliffs, and when he dropped his huge spear it struck the ground like a meteorite hitting the earth.

Its also speculated that the avatar encountered by Aegwynn was one of many, an avatar of Sargeras, but generally speaking it is referred to as "the avatar of Sargeras" in the singular form.