The Beginning.

In the Beginning, There was only one Universe. Undertale. It was the same as The Undertale of the original Universe. W.D. Gaster was stuck outside of it. Unable to interact with it, but able to see the events. This remained until he made three universes. The First one was Alphatale. The Next, The Balance. The balance was home of characters known as "Error 404s." Then, he made Puretale, an almost direct copy of Alphatale. Puretale was overcome by an Infection, turning into Infected!Tale. The AU's inhabitants became obsessed with destruction. The Three AUs warred, until The Balance and Alphatale put aside their differences, using their combined forces to hunt down all the inhabitants of Infected!Tale. Only one remained, that being Infected!Sans. A few remnants of Alphatale and The Balance also remained. Those beings Alpha!Sans, Alpha!Papyrus, Alpha!Frisk, Error404!Sans, Error404!Papyrus, and Error404!Chara. These 6 entered battle with the vengeful Infected!Sans. They fought for awhile. They realized they could never kill Infected. So they sealed him into The Abyss. Alpha!Sans used his own body to power a machine that would keep Infected!Sans sealed. For Billions of years. The Inhabitants of Undertale grew old in this time, and died. Gaster being the only one left. He decided to make more AUs. He made many, hundreds, Thousands. The Memories brought him happiness. However his cynical nature remained. Soon, he realized not all inhabitants of These Universes were content to simply live their lives, and attacked other AUs. So he made Error!Sans to fix this. However, Gaster found him to be an absolute abomination, a failure. He left him in a cold forest to freeze to death. However, he was saved by some travelers who took pity on him. Eventually, After Error realized the truth. He felt pointless. Pointless to such an extent, looking out on reality. That it drove him mad. So he concocted a plan. If he were to destroy all existing beings. He would have importance.