Zemouregal's memory is a book found during the Mahjarrat Memories miniquest. It can be found at the very end of Zemouregal's Base in the wilderness, in the map room. Note that you do not need to actually enter the Wilderness to get there, you can instead follow Hartwin in Varrock Palace. You need insulated boots and a Macaw familiar to reach it. When Zemouregal's memory is brought to Kharshai, he transcribes it into the book.
They say that Zamorak was banished by the other gods for daring to strike down one of their number, but this is a lie put about by the minions of Saradomin and Armadyl. They have not the strength to perform such an act, and any of them would have eagerly killed Zaros given the opportunity. The truth is very different. Not everyone who joined the rebellion did so for the cause of chaos itself. Drakan was promised the whole region of Hallowvale. Lucien was promised Forinthry. The Tsutsaroth who ruled the Avernic demons were not so easily swayed. Zamorak offered to help them throw off the shackles of their oppressive enslavement to the Chthonians, and still only Thammaron and Zebub had the wit and Imagination to actively fight for the rebellion.
Once the battle was won, and Zaros's shrivelled remains lay at Zamorak's feet, the Tsutsaroth saw no reason to wait. Zamorak, though visibly coursing with the divinity of our former lord, collapsed to the ground in agony and exhaustion. With the death of Zaros, the surviving loyalist Tsutsaroth sought direction from the victor as was their custom, and Thammaron gathered them around Zamorak's body, opening a portal to Infernus. Between them, the mighty demons were able to lift the new god, and they carried him out of Gielinor. Moments later, those fools Armadyl and Saradomin appeared, witnessed the carnage, claimed the fallen staff and stone, and vanished again. Denied our leader, the rebel army fell back from the capital, although many former Zarosians flocked to our banner as news of what had occurred spread. Azzanadra led a furious assault that drove us out of the city and across the empire, but could not marshal enough forces to finish us off.
Many demons remained in our legions, and through them, we learned of Zamorak's fate. He was on Infernus, using his new power to lead another rebellion, this time for the Avernic against their Chthonian masters, and he was winning. When his task was complete he would have earned the undying loyalty of an entire world of demons, and Gielinor would easily fall next. Many Avernic and some Mahjarrat joined him on Infernus, while those of us who remained held out against the Zarosians. We were aided in this as the other gods chose this time of crisis to launch their own invasions into the stricken empire, and Azzanadra's rapidly depleting forces were fighting on all borders.Nineteen years later, Zamorak returned to Gielinor at the head of an Avernic demon army even larger than the one Zaros had invaded with, although admittedly not as well-disciplined now that the Chthonians had been exterminated or exiled to the Abyss. He was flush with his new divinity and his success on Infernus, and demonstrated his valour by declaring the entire former empire his domain and attacking almost all of the invading gods simultaneously, along with the remaining Zarosian loyalists. The God Wars then began in earnest.