The god of the underworld in ancient Egyptian beliefs is Osiris. He is associated with the afterlife, resurrection, and fertility, often depicted in a mummified form. Osiris plays a crucial role in the Egyptian belief system, where he judges the souls of the deceased, determining their fate in the afterlife. His resurrection after being killed by his brother Set symbolizes rebirth and the cyclic nature of life. Osiris's mythology emphasizes the importance of moral behavior and the hope for eternal life, reflecting the Egyptians' profound beliefs regarding death and the afterlife.