Anubis was the most powerful and dangerous Goa'uld in history. Once a powerful System Lord, Anubis was banished by Ra and the others because his actions were deemed unspeakable, even among the Goa'uld. He was believed to have been destroyed, but managed to escape and trick Oma Desala into helping him to ascend. The Ancients did not want the Goa'uld to join them on the higher planes of existence after learning of his trickery, and forced him into a state between mortal existence and ascension.
Because of his knowledge of the Ancients, Anubis amassed unimaginable advanced technologies. He gathered strength for hundreds of years before returning to take his revenge upon the System Lords and dominate the Milky Way galaxy, before his permanent defeat in the Remnant Wars.