The Underworld, also known as Duat or Tuat by the ancient Egyptians, and Hell in the Christian religion, is a place of eternal punishment. It first appeared in The Mummy Returns, The Scorpion King 2: Rise of a Warrior and could possibly appear in The Mummy: Rise Of The Aztec; however, it's quite possible that this is just canon, so whether this is true or not is unknown.
The souls of those that were condemned in the afterlife as well as those that had committed severe blasphemies, were condemned to the Underworld, such as the High Priest of Osiris, Imhotep, who had attempted to bring his deceased love, the concubine Anck-Su-Namun, back to life by performing a proscribed ritual using the Book of the Dead.
Historian Herodotus, had discovered a concealed portal to the Underworld, within Minotaur's Labyrinth and that an intruder can only live upto an hour in the Underworld, before they turn to stone.
The Underworld was accessible in the Oasis of Ahm Shere, where the golden pyramid located in the centre of the Oasis had its stone floors cracked, revealing the Underworld as a deep, fiery chasm filled with dark savage figures that reached out at those that walked close to the cracks. The Scorpion King, within his pyramid had been defeated at the hands of Rick O'Connell and sent back to the Underworld; his defeat prompting the pyramid and its surrounding oasis to be taken back into the Underworld as Anubis took back his gift to the Scorpion King. In the midst of the pyramid's removal, O'Connell and Imhotep fell into the crack where the dark figures were, and both men held on as the souls pulled them into the abyss. O'Connell's wife Evelyn saw as her husband fell into the pit and was told by O'Connell to leave and take their son to safety, but Evelyn went back to help O'Connell out, while Imhotep called out to Anck-Su-Namun for help, but Anck-Su-Namun did not listen, running away instead and leaving Imhotep to hang onto the ledge himself. Imhotep, being despondent at his abandonment, gave up his life and released his hold on the ledge, allowing the souls to take him into the abyss. It is likely he remained in the Underworld, or when the pyramid was destroyed he was to.
Mathayus, at the age of 19, swore to end Sargon's unruly in Akkad, and went out to reach for Spear of Osiris to defeat the servant of Black Gods, Sargon. However, during his voyage to Egypt, he was notified by Ari, that the Spear of Osiris is only effective against Egyptian monsters and he requires the Sword of Damocles, which is preserved in the Underworld which can be only be accessed through a portal located within Minotaur's Labyrinth in Knossos, Greece. Mathayus, Layla and Ari journeyed to the Minotaur's Labyrinth, where they were accompanied by the Illyrian Fellowship to defeat the Minotaur. The companionship reached the portal to access the Underworld, by reading an ancient inscription in the portal. The Underworld consisted of three regions, namely: Desert, Swamp of the Dead and Astarte's Chamber, which possessed the Sword of Damocles under her guard. However Mathayus, Fung, Ari and Layla managed to escape with the Sword while the Illyrian Fellowship were unable to make it.