Disgaea: Hour of Darkness/Bonus Worlds
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A number of extra regions can be opened via proposal in the Dark Assembly. In order to do so you must have unlocked the proposal, have the proper amount of Mana available, and have enough influence with the Assembly (or be able to grind them beneath your boot-heel).
Prinny Land
Mana: 400
Cave of Ordeal
Mana: 700
Human World
Mana: 1200
Alternate Netherworld
Mana: 3000
The Mysterious Seal
Mana: 5000 Influence: 5000