Michael Morningstar, also known as Darkstar, was a serial killer and one of the protagonists' longtime adversaries from the canon. He made an appearance in Ben 10: Heroes of Evolution.
At an unknown point in time prior to the events of the series, Michael Morningstar's mind began to deteriorate until his quest for energy turned into a thirst for blood, and the instinct to commit actual manslaughter began to take hold. Michael started to acknowledge the harm he was doing and sought out to get help.
Michael went to several therapists and explained the necessity of a few liberties to be taken during their sessions, including the secrecy of his confessions (he would confess so information from his murders could serve as a basis to explain his instability). Most therapists refused, and Michael, overcome with both anger at their ignorance and paranoia of their potential reporting his crimes, would kill them. However, one Dr. Richard Wilson from Bellwood's St. Paul's Clinic was able to empathize with Michael's metaphorical mental torture and understood the intimacy of the situation.
For weeks Michael would talk with Wilson about his murders and his progress in teaching his mind to avoid his homicidal instincts - Wilson was clearly unsettled but never reported any of the confessions, feeling that despite a gradual approach to their goal resulting in several deaths, the gradual approach was the only way to go. Michael became grateful and formed a bond with Wilson, maintaining his own patience and never considering him as a potential victim.
However, Michael's mind was in fact wavering outside of the clinic, and while Wilson's understanding induced Michael's patience, Michael's homicidal intent seemed to increase and decrease rather than simply die down. Wilson saw this during a session where Michael inadvertently absorbed a small bit of energy from Wilson's right hand in a handshake. Wilson became slightly paranoid and secretly kept a recording of the session.
Ben and the cops were put on Michael's trail following a murder matching a very unusual M.O., which Ben was able to identify to Captain Rozum and Detective Blake. The same day, Michael killed six people in an apartment, considering suicide afterwards, but was unable to do so.
In a subsequent session, Michael solemnly visited Wilson and informed him that he had come up with a solution to solve his ways, but it relied on an overdose of killing instead of a gradual reduction, as the instincts acting on his mind need to be "deceived into an illusion of power". Wilson is horrified, but Michael angrily leaves, killing the people in the lobby on his way out.
Michael was traced the following rainy day to an intersection on Kelley Street, and Ben accompanied the cops there. Michael ran over a man on the road in his out-of-control car, and engaged in a violent confrontation with Ben and the cops - Michael escaped and overran several cops on a street further down who were waiting to ambush him.
That night, Michael broke into McDuffie Penitentiary where Wilson was incarcerated following his confession to the police. In an ensuing riot, an unstable Michael caught a glimpse of Wilson escaping and chased him out of the prison and down to a street. Ben and the gang arrived late but followed some footprints leading in the direction Michael ran off.
Michael cornered Wilson in a parking garage's upper floor and told him that he wasn't there to kill him - Michael went on to explain that he intended for Wilson to kill him, since suicide was something he couldn't do. Michael was able to suspend Wilson's objections through a long explanation on why it was the right thing to do, but Ben and the gang interrupted, making Darkstar livid.
Towards the end of a chaotic fight, Michael kept Ben, Gwen and Kevin overpowered, explaining his intentions to them and why stopping it would ultimately fail them. Ben countered with a statement that Michael's ideology did not extend to them and they needed to do what was right. Nearly dying, Ben grabbed Michael's hand and forced him to backfire - however, this resulted in Michael imploding from instability - this killed him.
The cops arrived and carried Michael's body out of the wreckage - Ben and the gang were horrified to see their longtime adversary dead. Michael was cremated under a tombstone reading "do people change?"