A Guide to Undertale AUs (old)
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Comycatdarkangel will be taking over maintaining this list. Find their post here: A guide to Undertale AU's 2
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Every single character in Undertale is changed. AUs that only affect some of the characters are not found here, nor are AUs that change only during the time Frisk is within the underground.
Basic Change (<-THIS IS A LINK)
The basic theme of Undertale is changed, flipped, or modified while still remaining in the same basic story in some which way or form. Most of these AUs have their own versions of the characters. It usually follows a specific theme,