Osiris, also called the Lord of the Duat, was a member of the First Civilization, revered as an Egyptian god of the afterlife, the underworld and death. He was also the consort of Isis.
In legends, Osiris and his wife Isis watched over the Egyptian pharaohs, with Isis using a Piece of Eden to keep them from harm. However, Osiris died after some time, murdered by his brother Seth, causing Isis to be overtaken by grief and despair.
She then attempted to revive Osiris by using the Ankh, but it would last for only one night. From the sexual intercourse between the two lovers that night, their son Horus was conceived.
Whenever Bayek would kill a member of the Order of the Ancients, he would tell them that Osiris awaited them so as to answer for their crimes against humanity.
Historians during the classical antiquity viewed Osiris as the Egyptian counterpart to Aita, Hades, and Pluto as they were all major chthonic deities. Serapis was a syncretism formed by these cultures.