The Alternate Netherworld is a bonus area in Disgaea: Hour of Darkness and its remakes. The Alternate Netherworld is the homeworld of the Alt Overlord and is ruled by Overlord Priere, although it is implied that it was ruled by Baal prior to the events of Hour of Darkness.
The Alternate Netherworld can be unlocked by passing the bill "Alternate Netherworld" in the Dark Assembly. You need a rank 8 character and 3000 mana in order for the bill to be proposed. The Senators of the Dark Assembly hate this proposal. As such, it is almost impossible to pass this bill without choosing the "persuade by force" option and defeating the opposing senators in battle.
Completing the final map of this area result in recruiting Priere as a playable character (not in Etna mode). It is also one of the requirements for unlocking Baal Castle, along with clearing the Beauty Castle bonus area.