Evil Energy

Evil Energy is part of the plot of Mega Man 8. When it first appears, it looks like an evil purple-looking robot whose name is unknown. In other parts of the game, it looks like a purple skull hologram that occasionally does a sort of fireball swirl. The energy absorbs evil within a person's mind and then multiplies to infect others--mainly robots, in the case of the game. Duo hunts down this energy to stop it from infecting the whole universe. It makes a minor appearance in Duo's ending of Mega Man 2: The Power Fighters, where he takes more Evil Energy that he found on Earth with him when he leaves.

In Mega Man 8, the first four Robot Masters carry the Evil Energy, although it isn't known if it had augmented their powers or bad personality, or just simply a power source.

This energy was the basis for the Maverick Virus, according to Rockman Zero Collection official site. (Translation in The Mega Man Network).