In Ancient Egypt, who was the 'God of the Underworld'?

Osiris was classically depicted as a green-skinned deity with a pharaoh's beard, partially mummy-wrapped at the legs, wearing a distinctive atef crown, and holding a symbolic crook and flail. He was one of the first to be associated with the mummy wrap. When his brother, Set, cut him up into pieces after killing him, Isis, his wife, found all the pieces and wrapped his body up, enabling him to return to life. Osiris was at times considered the eldest son of the earth god Geb and the sky goddess Nut, as well as being brother and husband of Isis, with Horus being considered his posthumously begotten son. In the Old Kingdom (2686 - 2181 BC) the pharaoh was considered a son of the sun god Ra who, after his death, ascended to join Ra in the sky.