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the mana goddess created life. living creatures took her power and used it to fuel their wars, and that wound up more or less killing the mana tree. it's clinging on for dear life, but it can't offer the support it once did.

the PCs are all souls from other lands (branches of the tree) where the dying tree could no longer provide mana and so they died, but the mana goddess managed to save them and kind of sort of revive their souls. so everyone's living in this town/city/thing and everything is fine and nothing ever changes... until people find an artifact with some remnant of mana, which makes them start remembering who they were

and then the whole game is people working toward reviving other lands (in-game that would be reviving the lands from Legend of Mana) and the endgame goal is to either bring back their initial world... or, you know. not. because is the mana tree even *worth* saving if all mana does is bring war and destruction?