What place did the underworld have in Egyptian mythology?
The Egyptian underworld was called Duat, though some argue that it wasn't really an underworld, but a pathway to the stars.
Egyptians believed that, after you died, your spirit would journey through Duat to see Osiris, the dead king of the netherworld. If you had money, your family would build a tomb and have you mummified in order to protect your spirit. If you had a lot of money, or were a pharaoh, you could have constructed (before your death) a pyramid, of which there are over 90 in Egypt, as a tomb.
The way to Osiris was perilous; you would have to contend with monsters and traps. The Egyptian Book of the Dead outlined how to make it through the various traps and trials that you would encounter in the afterlife, as well as hymns and spells that the living would use to help you along your way.
Once you reached Osiris, you would have to declare yourself guilty or innocent of various crimes. Then, the jackal-headed Anubis would weigh your heart on a set of scales, with a feather from the goddess of truth as the counterweight. If the heart weighed more than the feather, that meant that you were guilty, and you were eaten by Ammit, the aptly named Devourer of the Dead. If you passed, you were allowed to join the gods in the Field of Reeds — Egyptian heaven.