Nullifier

The Nullifier is a handheld device from the Mobius: 25 Years Later timeline. When Knuckles became a living Chaos Emerald again and started to remake Mobius to the way he thought it should be (including removing Julie-Su's bionics and making her wholly organic), Sonic sought to use the Nullifier to return Knuckles to normal. (Knuckles later claimed that Sonic didn't understand the Nullifier when he tried to use it.)

The Nullifier activated prematurely, removing Knuckles' Chaos powers and turning him red again, but also damaging his eye so that it had to be replaced by a bionic prosthesis.