The Yellow Devil is a confirmed stage hazard/boss battle present in the Wily Castle stage, from the Mega Man series of games - for this specific incarnation, the original Mega Man. (Please, add a picture if you see this article.)
The Yellow Devil can be spotted in the background on top of Wily's castle, and at an unknown point during the battle, will start seperating into chunks (as is it's usual battle strategy) and flying into the sky. Afterwards, these chunks will land in order onto the stage, reforming the Devil. It will then fire beams at the players and seperate into chunks again, moving to the other side of the stage and damaging any player that touches said pieces. This pattern will continue until it is defeated - this is difficult as it only spends a short period of time vulnerable and it's only weak point is it's eye. Eventually, after it is defeated, it will cause a massive, slow, extremely powerful Smart Bomb-esque explosion that counts as an attack for whatever player landed the finishing blow.