Osiris was the first son of Geb and Nut and the brother of Set, Horus (the elder), Isis, and Nephthys. He was one of the most prominent gods of the Heliopolitan Ennead, but his worship pre-dated the development of this fairly complex philosophy. Although Atum was installed as the head of the Ennead by the priests of Heliopolis, Osiris was considered to be the king of the underworld, and is the only deity who is referred to simply as “god”.
He was the great god of the dead, who once possessed human form and lived upon earth. After his murder by Set, Osiris became the king of the underworld and presided over the judgment of dead souls.
In order to enter his kingdom, the deceased had to undertake a perilous journey (aided by spells and amulets) to the hall of judgment where their heart was balanced against the feather of Ma’at (justice or balance).