Seth was jealous of his brother Osiris's power in Egypt and began to make a plan to kill him and takeover as king of Egypt.
Seth had a casket made to fit Osiris perfectly. He tricked him into the casket by saying that whoever fit into the casket could keep it.
Seth threw the casket into the Nile. Isis was very upset when she heard about her husband's death. She searched everywhere for the casket until she found it.
Seth was angry when he found that Isis had saved Osiris' body. He cut Osiris into 14 pieces and scattered his body throughout Egypt.
Isis and her sister Nephthys searched everywhere for Osiris' body. When they found all 14 pieces they sewed his body back together, and wrapped it in linen so Osiris became a mummy. Isis used powerful magic to bring Osiris back to life, but he could not return to earth because he was now king of the Underworld.
Osiris told Isis not to worry, that she would give birth to their son Horus, and he would become the king of Egypt as Osiris' legitimate heir. Seth was banished forever.