In Egyptian mythology, the god who personifies chaos and is the brother of Osiris is Seth.
Seth, also spelled Set, was associated with deserts, storms, violence, disorder, and foreigners. He was the son of Geb, the earth god, and Nut, the sky goddess; his siblings included Osiris, Isis, and Nephthys. In the Osiris myth, Seth murdered and dismembered his brother in a struggle for power.
Osiris's wife and sister, Isis, recovered and reassembled his body, and their son Horus later challenged Seth for the throne. Their conflicts symbolize the tension between disorder and rightful kingship, though Egyptian religion did not treat Seth as purely evil in every period. He also had a valuable protective role, fighting the serpent Apep alongside the sun god Ra during the sun's nightly journey.
Seth is commonly confused with a simple devil-like figure. His reputation changed over time, and some Egyptian traditions honored him as a powerful defender. The spelling “Set” is closer to modern Egyptological usage, but “Seth” is a standard English form and is the canonical answer here.