Born the son of Geb (the earth) and Nut (the sky), Osiris was the twin brother of Isis and became the first pharoah. He taught mankind the secrets of farming and agriculture, and according to Egyptian myth and legend, brought civilization itself to the world. Ultimately, the reign of Osiris was brought about by his death at the hands of his brother Set (or Seth).
The death of Osiris is a major event in Egyptian legend. In one variation, Set became jealous of Isis' love for Osiris, and plotted to take the throne. Set ordered that a beautiful chest be built and painted, and announced that it would be a gift for whoever could fit inside it. Naturally, Osiris hopped in and saw that it was a perfect fit for him. Set promptly shut the chest and sealed it, preventing Osiris' escape, and dropped it into the Nile, where it floated out to sea. Isis, heartbroken at the loss of her beloved husband, searched the world over to find Osiris. Finally, she located the chest and resurrected her lover with her magic, long enough to allow for the conception of Horus, who later exacted revenge upon Set.
In other versions of the tale of Osiris, the chest washes up on a shore, where it is enveloped in the trunk of a giant tree. The tree is later chopped down to be used as a pillar in a king's palace. Isis goes to the palace, where she begs the king's wife to allow her to free Osiris from the pillar. This she does, fanning breath back into him with her wings of magic.
Following Horus' conception, Anubis embalmed the body of Osiris, thus creating the first mummification process. Later, Set found the body, tore it to pieces, and sent the various body parts to be buried in different locations around Egypt. After his death, Osiris became a guardian of the underworld, or Duat. He became known as the judge of the dead, and it was his job to determine if a traveler's soul was worthy of entering the kingdom of paradise.
In some parts of Egypt, Osiris came to be known as a harvest god, as the cutting and dismemberment of his body was associated with the cutting and threshing of grain.