He brutally murdered his father in front of the other Goa'uld in around 22,000 B.C., taking his father's troops as his own. He then decided to divide them equally among the strongest of his kind to solicit their stewardship; this ensured a kind of peace among the Goa'uld and also meant that none could ever become powerful enough to challenge any other, as mutual destruction awaited any who tried.
Apep soon declared his underlords as his 'brothers' and 'sisters', remaining firmly in control yet giving each of them equal rights in rulership. Each underlord pledged loyalty to him and so Apep held rulership over many of the future System Lords; Nut, Thoth, Ra and their servants Bastet, Isis, and Osiris. For a time, the Goa'uld were united under a single rulership. They swept across the galaxy, their numbers swelling enough to almost wipe out the Unas for a second time as too many larval Goa'uld were bred. Somehow during this time, the average lifespan of the Goa'uld was significantly lengthened which only caused further population problems; because of these, Apep continued the ancient practice of ritualistic cannibalism.
This pseudo-peace was not to last, however, as soon another Goa'uld came into power around 19,000 B.C. with victories too numerous to count; Anubis was born. He also pledged his support to Apep and was brought in as another of Apep's underlords as the Lord of Death due to his ruthlessness. Apep remained wary of his 'children', as he saw greed in their eyes which could threaten the fragile balance he had created; despite this, he was doomed to failure. The Goa'uld, under Apep, spent the next thousand years mapping out most of the stargate network and building up vast fleets through the Ancient technology they discovered. Due to the fact that they did not find the original owners of their spoils, they soon carved up the spoils amongst themselves; though a few species dared to stand against them, they all fell to the power of the Goa'uld.
Around 17,800 B.C., after Thoth's invention of the sarcophagus which introduced naquadah into the Goa'uld's bloodstream, Apep started to falter as a leader of his people; his underlords became more and more daring and Tiamat was welcomed into the fold with others being cast out and destroyed. It was then that Apep started to show the signs of madness; the end of peace within the Goa'uld was near. Anubis solidified his position with Apep, becoming his right hand man who was often found at his side. This caused dissent among Apep's original 'children', including Ra, who looked upon Anubis with disdain as he was favored like no other; great gifts and boons were given to him for seemingly no reason.
Anubis spent the next three hundred years becoming closer and closer to Apep, and eventually was able to solicit information pertaining to a great cache of Ancient weapons from Apep which he had been hiding from his other 'children' as they were so powerful that if any Goa'uld was able to access them then they would surely destroy the fragile peace he had built. Anubis was able to learn this location through forcibly exposing Apep to Thoth's healing device, driving Apep further and further from sanity.
Anubis, determined to take leadership of the Goa'uld for his own, made his own bid for power by utilizing a "superweapon" found in the cache of Ancient weapons. His plan partially worked as he was able to capture Apep, removing and devouring his symbiote before the other Goa'uld, declaring himself Emperor. His power would not last, however, as Ra and the other Goa'uld rallied in Apep's name and drove him from power; this would be the first time in their history that the Goa'uld acted with nobility. (RPG: "Living Gods: Stargate System Lords")