Name: King of Mutants, or simply King.
Real name: Bryan McDonald
Powers and skills: King is one of the most powerful mutants in existence, his strength rivals that of the Hulk and the Juggernaut. His powers include superhuman strength and durability, energy projection (blasts), and flight. He is also an effective leader, strategist, and tactician.
Personality: He is a complete sociopath. He manipulates his followers and uses all sorts of means to control them. He is also extremely sadistic, not stopping until all who stand against him are wiped out. He sees himself as the savior of mutants, and sees humans as inferior and seeks to launch a genocide against them; creating a utopia in which only mutants exist. He often acts on impulse and is extremely psychotic, showing signs of Bipolar disorder and megalomania; although this is his greatest downfall, it also makes him an extremely dangerous and unpredictable adversary for the X-Men.
I can't really come up with a good origin story, so I'll just get into the gist of it: Bryan McDonald is a 25 year-old omega level mutant who has been an outcast for most of his life. Having witnessed the events of M-Day and AvX, he sees both the X-Men and Magneto as weaklings and sellouts who have failed in their primary mission to protect mutant kind. He forms a group of other mutants, calling themselves the Covenant: A cult like group of mutant outcasts, who see Bryan as a King and worship him. In his first official appearance, he attacks Utopia, defeats all of the X-Men almost singlehandedly, and beats Magneto within an inch of his life. He declares himself to be the King of all mutants and gives the X-Men two choices: That they serve him or die. Some such as Hope Summers, who at this point has become disillusioned with the X-Men, join him. but the core X-Men such as Cyclops, Wolverine, and the others outright refuse to join him. So instead of killing them, he expels them from Utopia. He eventually gets his hands on nuclear missiles, aims them at various major cities, and orders that all of the world's governments relinquish power to him or suffer the consequences.
This leads to the Avengers and all of Marvel's other heroes teaming up with the X-Men and invading Utopia. After a brutal battle, King and the Covenant are defeated. Hope Summers and King fall in love, and King makes her his queen. King, Hope, and the rest of his followers are able to escape to an uninhabited island in the Pacific Ocean where they regroup. Vowing to return and annihilate both the Avengers and the X-Men.