"It'd offer an insufficient flow of mana power," Zetta said, apparently finding this question the most reasonable thing this guy has asked the entire conversation. "An Overlord and a Netherworld exist in a sort of symbiosis. Their powers are connected. This train, even given all its special stuff, doesn't have anything on an entire world for potential power. It also helps significantly to have lots of living creatures in the Netherworld who accept or at least don't actively protest the Overlord's rulership. My own mana power may be immense, but without the ability to write in the Sacred Tome I can't translate it directly to reality, so I need a strong mana flow to fix this. Maybe even several."