Xorn
A new character introduced during Grant Morrison's run on the X-Men, Xorn was a Chinese mutant who had a star for head, giving him the ability to manipulate energy and gravity.
Except there really was no Xorn - it was a false identity made up by Magneto, who used it to infiltrate the Xavier Institute and destroy it from the inside. Magneto tried to lead the mutant revolution, but ultimately failed. It all wrapped up nicely and is recommended reading.
But it didn't end there. Marvel decided to keep Xorn as a character, so they introduced his brother, who looked the same as Xorn and had the exact same powers. And that the original Xorn was not a false identity created by Magneto, but a real mutant who was under the control of an evil, powerful force. This tacked-on plot development did not go over well, and the Xorn characters have quietly gone away.