The Genesis Wars
During the first years of creation, Horus ruled the living plane, Osiris was given the Underworld, and Anubis, the oldest brother, was to guard the gate in between.
Through Osiris' corrupt nature, greed, and jealousy, he assassinated his younger brother Horus out of spite so he could take over the living world. However, Anubis stopped him, and in his rage, casted Osiris back to the Underworld along with his army through a long campaign that lasted many, many years. When the first war was over, Anubis took over Horus' role, and his own, accepting responsibility for not being able to save his younger brother, Horus. In his mourning and greiving, Anubis became corrupt and lost his way, and became evil, and more powerful.
After hundreds of years, Osiris had been secretly building another army, and once again, attempted to overthrow the throne to the living plane. However, just like the first time, Anubis retaliated, and they were met at a stalemate.
Anubis' general, and daughter, Setko, was persuaded and convinced by Osiris to flee the war and escape her father's reign in hopes he could trick her into giving him her powers, her abilities being much like Anubis', and she fled before the last battle of the second Genesis War. Because of her sudden escape from her father's grasp, Anubis' army was crippled, and Osiris found his advantage. However, after a long and terrible last battle, neither could claim victory, and both brothers lost more than before. In anger, Anubis sent out several of his finest hunters after his daughter for treason. But because she used her ability to travel through time, killing her in the process, she had fled far into the future by thousands of years. Because of this, only few of Anubis' hunters remain to find her.
After Anubis sent out his hunters after his daughter, Osiris too set out a few of his beest soldiers to find her as well to take her powers so Osiris could have the upper hand, and once again, attempt to overthrow the living plane. But because of her time travel, only a few of his soldiers remain.
Both of the gods accepted defeat, and both went dormant for many, many years. Because of this, the timeline was corrupted, and many wars have been waged since then. Chaos ensued, which is exactly what Ra, the creator and father, was afraid of.