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 absorb 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.

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.

Trivia
Some fans speculate that this energy was the basis for the Maverick Virus, although no official sources confirm that theory.