UnderVoid

UnderVoid is an AU were Gaster finally was able to return to his timeline via unknown methods. However, when he returned he found out parts of the void he was trapped in began to seep into his timeline and before Gaster or Sans could stop it the void consumed and corrupted the timeline, warping it into an abyss-like Underground. Many of the characters there now are sadistic and maniacal versions of there former selves.