Hell

Background
At the bottom of the multiverse stands Hell, a place where time does not function, and souls are sent to be punished.

Hell's original purpose was to hold Demise and prevent him from escaping into the Dark Dimension. Zero gave Core the power of ruling Hell, and Core later passed it off to Flame.

Once in Hell it is normally impossible to leave, but is possible under two circumstances. 1) Having a key to enter Hell, 2) Granted permission from Zero or the ruler of Hell.

Due to Mason creating a key to Hell, this was how Midnight was able to enter Hell and be taught by Demise.