Little is know about Saradomin, but it is confirmed that he came over to Gielinor when Guthix was sleeping. His obsession with birds lead to find the Aviansie, and later Armadyl. He fought with Armadyl against the evil Zamorak for a while, but Armadyl became distraught and left Saradomin to take of Zammy alone. Saradomin lacked the with and knowledge of Armadyl, Zamorak, and pretty much everyone else in this universe so he just led his armies to destruction. If Guthix never woke up then Saradomin's army would have been completely destroyed by evil.