Osiris is the god of the underworld. He is shown as a mummified pharaoh, sometimes with a crown with a pair of rams horns at its base.
Osiris was second only to his father, Re, and was the leader of the gods on Earth. He was the husband of Isis and the father of Horus. Osiris lives in the underworld as the lord of the dead, after being killed by Set.
After Osiris was killed, Isis resurrected him with the Ritual of Life. This was later given to the Egyptians so that they could give eternal life to all their dead.
In the underworld, Osiris sits on a great throne, where he is praised by the souls of the just. This part of the underworld is like the land of the living, but without sorrow or pain.
Osiris has to remain in the underworld because the dead may not return to the land of the living.