"Osiris is considered one of the principle Egyptian Gods. He has been referred to as 'he who dwells in Orion with a season in the sky and a season on earth,' among other epithets. Whatever his identity, most ancient writings strongly point to his extra-terrestrial origin." (PoL)

Brother of Seth, Brother and husband of Isis, Father of Horus, Son of Ra.
Osiris was one of the children of Ra, the supreme deity. Ra divided his kingdom in Lower and Upper Egypt, and gave Osiris rulership over Lower Egypt, Seth over Upper Egypt. Seth however schemed to overthrow Osiris, and eventually Osiris drowned, after having been castrated by Seth. Isis, however, managed to raise Osiris from the dead. Through some magic Isis and Osiris got a son, Horus. Osiris left Earth to join his father Ra in his heavenly abode. Horus became the ruler over a reunited Egypt.

Osiris (Ausar)

The god of the dead, and the god of the resurrection into eternal life; ruler, protector, and judgeof the deceased, and his prototype (the deceased was in historical times usually referred to as "the Osiris"). His cult originated in Abydos, where his actual tomb was said to be located.
Osiris was the first child of Nut and Geb, thus the brother of Set, Nephthys, and Isis, who was also his wife. By Isis he fathered Horus, and according to some stories, Nephthys assumed the form of Isis, seduced him thus, and from their union was born Anubis.
Osiris ruled the world of men in the beginning, after Ra had abandoned the world to rule the skies, but he was murdered by his brother Set. Through the magic of Isis, he was made to live again. Being the first living thing to die, he subsequently became lord of the dead. His death was avenged by his son Horus, who defeated Set and cast him out into the desert to the West of Egypt (the Sahara).
Prayers and spells were addressed to Osiris throughout Egyptian history, in hopes of securing his blessing and entering the afterlife which he ruled; but his popularity steadily increased through the Middle Kingdom. By Dynasty 18 he was probably the most widely worshiped god in Egypt. His popularity endured until the latest phases of Egyptian history; reliefs still exist of Roman emperors, conquerors of Egypt, dressed in the traditional garb of the Pharaohs, making offerings to him in the temples.

SEE ALSO Anubis, Geb, Horus, Isis, Nephthys, Ra, Seth.