The Shimmer is believed to be a harbinger of imminent and inescapable death, and those who see it often suffer severe psychological breakdowns and depression upon the realization that they are doomed. Apparently never captured on recording devices, the Shimmer allegedly takes the form of a fast-moving greenish optical illusion.
The Shimmer has existed since before The Big Bang. Master of his own domain outside of existence, The Shimmer oversees the endless universes, searching for the reason for why Earth throughout the multiverses have become the universal hotspot.
A historical constant, The Shimmer has been present for every event on the multitude of Earth.
In Legend of the Seven Lights, The Shimmer was seen wandering around the Land of Heat and Clockwork, and its presence panics two gangs into killing each other. Vanellope speaks with The Shimmer as she asks how to get back to Earth, to which it's confused which Earth. It mentions to know Dillon Simmons before retreating, ignoring Vanellope's protests.