Osiris (Asir) 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”. This gives us some indication of his prominence and popularity.
The oldest religious texts known to us refer to him as the great god of the dead, who once possessed human form and lived upon earth. After his murder by Set he became the king of the underworld and presided over the judgement 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 judgement where their heart was balanced against the feather of Ma´at (justice or balance). It is important to distinguish this from the Christian view of judgement. The ancient Egyptians were a pragmatic people. A person was not expected to be perfect, just balanced. An example of this is the “negative confession” (in which the deceased listed all of the evil things he had not done during his lifetime, i.e. “I never murdered anyone”) which indicates that it was more a case of convincing Osiris that you deserved admission rather than passively awaiting judgement.