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The actual place is the Underworld realm
Night at the Museum: Kahmunrah Rises Again
The Duat, or otherwise known as the Underworld, is an afterlife realm-domain that appears in Night at the Museum: Battle of the Smithsonian and Night at the Museum: Kahmunrah Rises Again. It is also based upon the same location from Greek/Egyptian Mythology.
Appearances
In both Battle of the Smithsonian and Kahmunrah Rises Again, Kahmunrah uses the Tablet of Ahkmenrah to open the portal to summon his humanoid-bird warriors, which are named after Horus and Ra, in an attempt to concur the world, but his plan failed both times.
Night at the Museum: Battle of the Smithsonian
In Battle of the Smithsonian, Octavius had gotten Abraham Lincoln to help him out, which Abe does by scaring Kahmunrah's soldiers back into the Underworld. At the end of the battle, the portal was then sealed when the tablet was removed after Kahmunrah was pushed into the portal.
Night at the Museum: Kahmunrah Rises Again
While in Kahmunrah Rises Again, his soldiers run back to the portal and inside the Underworld after Nick plays a three-chord note backwards. With this, Kahmunrah and Nick fight, only for Nick to use a flashlight to get Kahmunrah sucked fully into the portal. The portal is then sealed after the tablet is taken out of it.