Mewni is an alternate dimension, that appears as the homeland of Star Butterfly, and of her royal family. A medieval land, bordered with several kingdoms, including the undeground Castle Avarius, Mewni has been frequent threatened by various evil forces, including the commander of monsters, Ludo, the scheming immortal monster, Toffee, and many others, though it met resistance from the Butterfly royal family, as well as from Butterfly's closest friends, and the dimension's inhabitants.

Regarding the villain wars, Mewni's existence is displayed by the appearance of Castle Avarius, one of the main kingdoms, during the events of the second Disney Heroes vs. Villains War. In the series, Castle Avarius is featured as one of the primary locations of the villain forces, although the Butterfly family, as well as the kingdom's subjects make an appearance, on different zone of the galaxy.

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