* MY MY, THIS IS THE SADDEST DISPLAY I'VE EVER SEEN, DARLING!
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The Anti-Void is a location originally conceived by CrayonQueen/LoverOfPiggies for ErrorTale, though it was latter adopted by the Undertale Fandom Community for the Multiverse. It's a Border layer of reality that exist as a gap between each and every single AUs in existence.
The Anti-void functions quite similar to the Void, but with a few key differences. For one it's pure white, like a blank page, as opposed to the Void's grey color. Another difference is that while the Void functions as a gap outside timelines, the Anti-Void functions like a borderworld between AUs. While the Void is a place where nothing exists, the Anti-void is a place where everything and anything exists.
It's unknown if many of the non-AU locations (like the Doodle-sphere and the Omega Timeline) are located in the Anti-void; as the only documented residents of the borderworld are Error and his "room-mates".
Staying in it for long enough will turn one into an Error.
Example: After Blueberry/Swap was left in the Anti-Void by Error!Sans, Swap eventually turned into Swap!Error