Osiris is the Egyptian god of death, resurrection, and fertility. He is often depicted in the formal garb of the pharaohs, holding the crook and flail of office, wearing the crown of Upper Egypt, and wearing the ceremonial beard. In most works of art, he is also swathed like a mummy, and his skin is green, symbolizing fertility.
Archaeological digs indicate that Osiris is one of the oldest recorded gods in Egyptian history, the son of the earth god and the sky goddess. Worship of him was centered at a temple in Abydos, although Osiris was widely venerated throughout Egypt, and even beyond; his cults were, at one point, quite fashionable in Rome, for example.
According to legend, Osiris was married to his sister Isis, and he was drowned in a trunk by his brother Set. In various legends, Isis found the trunk floating on the Nile, or she found the body washed up on shore, and she brought him back to life to father their son Horus. However, Set was enraged by this, and he tore Osiris to pieces, scattering the fragments across Egypt.
Isis painstakingly reassembled her husband, with help from others, and brought him back to life again, but by the rules of the Egyptian gods, Osiris was not allowed to dwell in the land of the living anymore, since he was considered dead. As a result, he was sent to the underworld, to watch over the dead and judge them when they entered. As a result, the dead came to be associated with Osiris, with Egyptians believing that the righteous would dwell in his kingdom after death.
Many depictions of Osiris show him in the underworld with Anubis, a god associated with the afterlife, and the scales of justice on which the souls of the dead would be weighed. Cults of Osiris used the imagery of death, rebirth, and judgment extensively, using the god to inspire followers with ideas of immortality for the righteous.
Because Osiris died twice and was resurrected, he is also associated with life and rebirth. Egyptian society was very focused on cyclical patterns of fertility and life, thanks to the annual flooding of the Nile, so it is perhaps natural that this god could represent both life and death at the same time for the Egyptians. The story of Isis and Osiris was acted out annually in many temples, complete with offerings, chantings, and storytelling, and it may have influenced similar stories in other parts of the Ancient world, such as the tale of Persephone.