|The following is based on Fallout: Brotherhood of Steel and has not been confirmed by canon sources.|
The rotten eyeball is a quest item in Fallout: Brotherhood of Steel.
The rotten eyeball is one of Harold's missing pieces that is found in the Bridge East area of Los. It can be found by crossing a set of pipes that lead off from the main path. For slightly better directions see Find Missing Pieces.
Bridge East - found inside of a footlocker.