Anubis

Anubis was an ancient Egyptian mage, who declared himself protector of the dead, thus later worshipped by Egyptian as god of death and rebirth.

Biography
Early life of Anubis remains as a complete mystery, beside our knowledge of him being a powerful Egyptian mage. Primairly, he and his people allowed the Avatars to remake the world to Utopia. But once Anubis realized that people lost their free will, decided to undermine Utopia. Knowing that Avatars are constantly watching, he sealed himself in ancient tomb (pyramid) and spent years trying to create potion stron enough to destroy an Avatar. Once he succeeded, he lured an Avatar to him and killed him, dying in process.

His death opened eyes for most of the humanity. They denied worshipping Avatars any longer and forced them to retreat, threatening them with Anubis's potion. Avatars brought the world back to its previous state, but promised to return.

Anubis' remains were burnt and confined in ancient urn, guarded by mage's fellow spirit, known as Guardian of the Urn. After this event, Anubis was worshipped by Egyptians as god of death and rebirth.

Speculation
Many fans believe that Anubis' next incarnation was Kyle Brody or that he became Angel of Death.