Nullifier

The Nullifier is an alien device of unknown origin that came into the hands of the Knothole Freedom Fighters of a possible future version of Mobius. Little was known of its workings, but Sonic felt that it was worth the risk to stop a maddened Knuckles from altering Mobius to his own ideals. The device was successful in stopping Knuckles, but cost him his left eye, something that he would hold against Sonic for years and point out as a possible cause of their world's deterioration. (StH: #140)
 * It is unknown if the Nullifier existed in the alternate timeline.