When we try to make some sense out of the many Egyptian gods and goddesses, we must keep two important facts in mind. First, early in Egyptian history lower (north) and upper (south) Egypt were unified under one ruler. This union resulted in the merging of several cultural traditions. Second, because ancient Egyptian civilization existed for more than three thousand years, the deities and myths gradually changed over time as a result of new ideas, contact with other peoples, and changing cultural values.
One of the best-known legends in Egyptian mythology, that of the god Osiris, revolves around a deity who at one time may have been a local ruler in the Nile River's delta. Originally he was a god
associated with the city of Busiris in the Delta and is an example of a regional god who gained countrywide acceptance.
According to the myth, Osiris was the king of Egypt who was killed by his jealous brother Seth. This evil brother then cut up Osiris' body and scattered the parts throughout Egypt. Osiris had a faithful wife Isis who, along with her sister Nephthys, gathered the pieces together. Using her magical abilities, Isis put the pieces back together, but Osiris could never again live like the other gods. He, therefore, reigned as lord of the underworld, while his son, Horus, became the ruler of Egypt (see below). Osiris is represented as a mummified king.
Because the legend told of Osiris' death and rebirth, the Egyptians honored him as the god of the dead. He is depicted as a mummy holding the crook and flail, the insignia of kingship. During the Old Kingdom (ca. 2750-2250 B.C.), he became associated with the deceased pharaoh in the afterlife. During the Middle Kingdom (ca. 2025-1627/1606 B.C.), when many of the funerary rituals became available to much of the population, all individuals became associated with Osiris upon their deaths.
http://www.egyking.info/2012/10/ancient-egyptian-gods-and-goddesses.html