In the mythology of the Chronicles of Darkness, Osiris is said to have been a mortal, king of Egypt, who was murdered and dismembered by his jealous brother Seth. He was brought back to life by his sorceress sister-wife Isis, and subsequently became Progenitor of a Lineage of Prometheans, who took his name as their own - the Osirans. Despite the fact that Isis could not recover Osiris' genitalia when she reassembled his body (it had been eaten by a crocodile), the couple later had a son, Horus, who went on to defeat Seth.
Legend has it that Osiris eventually ascended to divinity as a god of the underworld. How he achieved this - and even whether or not it ever happened - are open to question; the Osiran Lineage, however, generally take it as an article of faith.