The story is set in the world of Luminasia, where large countries dominate smaller ones for the sake of harvesting Hostia, a form of crystallized mana produced by the World Tree. The story follows the player and the guild Ad Libitum as they investigate a red smoke, capable of altering the forms of living creatures, which people have mistaken for the wish-granting manifestation of the Descender.

(Source: Aselia, the Tales Wiki)