Evil Energy

Evil Energy (also called Dark Energy) is an alien energy source that first appeared in Mega Man 8. Usually, it looks like a swirling purple flame in the shape of a skull. The energy absorbs evil within a person's mind and then multiplies to infect others--mainly robots, in the case of the game. Due to this method of replication, the original source can only be harnessed by beings of pure evil. Duo hunts down this energy to stop it from infecting the whole universe and possesses the ability to heal its victims if their minds are not completely consumed by evil.

Story
In the opening of Mega Man 8, Duo is seen in space, battling an evil-looking robot utterly consumed by the Evil Energy (it is unknown if this is another victim of the Evil Energy or an actual manifestation of the energy itself). He manages to defeat the robot, but both combatants fall to Earth, impacting a remote island. Unfortunately, the island happens to be Dr. Wily's secret base, allowing the mad scientist to discover the remaining Evil Energy. He escapes with his prize as Mega Man shows up, and eventually learns how to harness its evil powers.

Using the Evil Energy apparently allowed Wily to power up his robots and construct a massive new fortress in the heart of a volcano. His first four Robot Masters carried capsules of Evil Energy in their bodies, although it isn't known if it had either augmented their powers, manifest a bad personality, or was simply a power source.

When the first capsules of Evil Energy were collected, Mega Man brought them back to Dr. Light in hopes that his creator could analyze them. However, Duo, having been brought to Light's laboratory for repairs, sensed the Evil Energy and destroyed it before breaking out and heading for the source of the power-Wily's tower fortress. Pursuing the mysterious robot, Mega Man also arrived at the tower, only to be attacked by a massive robot powered by Evil Energy. Duo arrived and destroyed the robot with a single blow, before explaining the nature of Evil Energy and his mission to Mega Man. Since Duo had to leave and destroy the Evil Energy spreading around the world, Mega Man infiltrated Wily's fortress to stop his experiments with Evil Energy. Along the way, he was forced to defeat Bass, who had used the energy to power himself up and fuse with Treble.

When Mega Man destroyed Wily's final machine, the Evil Energy within it suddenly entered his body in an attempt to corrupt him. Duo spirited him away from the crumbling castle and managed to purge Mega Man of the energy through a combination of his own abilities and Mega Man's noble mind. With the last of the Evil Energy destroyed on Earth, Duo left to find other worlds where the evil power had manifested.

Evil Energy (or a similar energy) makes a minor appearance in Duo's ending of Mega Man 2: The Power Fighters, where he comes to Earth in search of new sources that have appeared since his last appearance. After defeating Dr. Wily, he collects the energy before leaving once more.

Trivia

 * It's often speculated that this energy was the basis of the Maverick Virus, but it's unknown if there is a connection between them and the Roboenza.
 * According to the content that was in the Rockman Zero Collection official site, Dr. Light made X with a perfect virus counter-measure because of an unknown computer virus of alien origin that caused robots to become violent and riotous. The virus mentioned is believed to be the Evil Energy. (Translation in The Mega Man Network).