8 X-Men Characters You Didn't Realise Had Died
7. Magneto
So right up front, let's make it clear. Yes, the version of Magneto being talked about here was technically Xorn in disguise as Magneto in disguise as Xorn. And yes, it is technically incorrect to say that Magneto died during this storyline. But when Grant Morrison wrote this story, there is little to no doubt that he meant to kill the real Magneto.
The storyline in question came towards the end of Morrison's stint on New X-Men in 2004. Magneto is revealed as the big bad who has been hiding in disguise just under the X-Men's noses as the superhero Xorn. He takes over New York City and turns it into an authoritarian deathscape. Magneto is ultimately killed by Wolverine and it seemed the master of magnetism would be out of continuity for awhile. So imagine readers' surprise when just a few months later Magneto was revealed as a main character in the new Excalibur series.
To explain this the writers ultimately retconned that Magneto was in hiding and the menace that attacked New York really was Xorn. Yes, it is terribly confusing. Some people may have been asleep at the wheel on this one, but the short of it is that Magneto was killed and resurrected so quickly even Marvel editorial seemed to forget he was supposed to be dead.