Killer Suggestion : TrueChaos

(True Chaos isn't related to Chaos from SA1) True Chaos's Real form is a Chaos Emerald, it is a Chaos Emerald that originated from an unknown world it is pure black like space itself

True Chaos used to be in a Dimension that was made just for it but True Chaos managed to find a way to escape and it found a world akin to our Sonic world and True Chaos began to spread it's energy corrupting those who come in contact with it slowly

Sonic is running along the green hill zone when he suddenly finds True Chaos and pick pockets it due to thinking it was just a normal Chaos Emerald and True Chaos starts to haunt Sonic until one day where Sonic is fighting Eggman and uses True Chaos alongside the nornal Chaos Emeralds to go super. This results in True Chaos fully possesing Sonic and using him to first kill Eggman, kill Knuckles so he wouldn't free Sonic by using the Master Emerald and shattering it and killing pretty much everyone except Tails whom he torments for years and in each encounter lets him live but not with out a few missing parts or too

A ring opens up to "his" world and outcomes İndieX who proposes him a deal "Come to my world and haunt your victims for eternity" True Chaos accepts it and to this day torments the Survivors daily with no Hint of boredom