Xorn

Warning: This article may be confusing.

Xorn was a mutant that was seemly sold to the villain John Sublime by corrupt Chinese officials. He wore a skull-like mask that supposedly block the energy emitted from his "star for a brain". To prevent him from killing himself, Cyclops (with the aid of the telepathic Emma Frost) offered to let him join the X-Men. Cyclops found him in a monastery, where they were kidnapped by aliens. They escaped and Xorn used his healing powers (the monks at the monastery claimed he had such powers) to heal the X-Men from a nanobot infection and fix Xavier's spine (which was the reason he couldn't walk). When he was on the team, he acted optimistic and somewhat naively. It was revealed his helmet blocked telepathy. He was put in charge of the "Special Class". In the "Planet X" story line,  Xorn turned out to be Magneto in disguise (he claims he was controlling the nanobots and used some of them to "glue" Xavier's spine to fake his healing powers). He conquered New York City. However, the combination his use of drugs, going insane and hypocritical acts (such as killing normal humans like a Nazi despite being a Holocaust survivor) turned his supporters against him. Wolverine chopped off his head, because he killed Jean Grey.
Marvel Comics wisely decided to make it so Magneto really didn't do that. A mutant named Shen Xorn revealed that John Sublime was using pyshic powers make his brother, Kuan-Yin Xorn, into pretending to be Magneto (pretending to be Xorn) and doing all the previously mentioned actions. Emma Frost read Shen's mind and revealed he was telling the truth. Shen lost his powers in House of M.
The Scarlet Witch accidentally turned millions of mutants' powers into a "big ball of mutant energy" (according to Joe Quesada the editor-in-chief) called the Collective. The Collective merged with postal worker Micheal Poster. He unintentionally killed the superhero team Alpha Flight. The "big ball" retained the original Xorn's personality (Quesada claims it was because of his "pyshcokinetic powers" despite his powers not being pyshcokinesis). It claimed it (when it was Kuan-Yin Xorn) took Magneto's image because he thought mutants would follow him that way. Despite various heroes destroying the Collective, Micheal retained the powers he had used and joined the Omega Flight.
My sources were:

• http://en.wikipedia.org/wiki/Xorn_(comics)
• Atop the Fourth Wall episode "Top 15 Worst Heroes Becoming Villains"