The Reality Tear Device

The Reality Tear Device is a device that tears a hole into the fabric of reality, it's influence extends to beyond the Undertale bubble. This particular version of the RTD was created as a proof of concept as it is too small for an adult monster to travel through.

Story
The Reality Tear Device was created by W. D. Gaster in the True Lab to find a reality where humans never trapped the monsters underground or never existed at all, it dragged Ace into the Akintale universe, permanently changing the course of history in this universe.

Pulling Ace into the world caused the machine to be destroyed with its spinning ring shredding itself and the bulb-like liquid container to explode. WD intended to repair the machine, but stopped after overhearing Ace explain how lucky he was to survive the experiment.

A few decades after Gaster's death, Sans found out about some monsters sneaking into the lab to look at the experiments and steal anything left of value. He retrieved the machine, concerned that someone may attempt to turn it on.

After a while, Sans became suspicous of humans having the reset ability but had no real proof. During Brendon's time in the underground, he received the first genocide nightmare and tried to repair the machine in hopes of contacting Ace. After a hundred or so years, Sans gave up on fixing the machine mainly because he didn't have access to the spare parts required to repair it.

Trivia

 * Apparently travelling using the RTD is a horrifying experience.
 * It is unknown exactly what effect it has but those who move through the tears are unconsious after arriving and become panicked apon waking up.
 * It is assumed the device shows different things to different people and that they experience different sensations depending on the worlds they travel between.
 * It is the only device that allows people to travel outside of the Undertale AU section without travelling through a crossover first.