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LustBerry. That's it.

Lust Sans and his universe Underlust terrify the others in the multiverse, even the Bad Sans'. But one night at a party Lust meets a sweet little berry, Blue Sans. They get along and, even though they just met, Blue offers Lust a place to hang out whenever he needs, Underswap.

/So fair warning, only Sans' and Papyrus' know about AU's. Frisk and Charas know as well but they're rarely in this if at all, sorry bout that. I hope you enjoy it!/