He didn’t even die there, if you can believe it. Though he is catatonic and carried home by Jean Grey. This is one of the few comic books I actually had.
Immediately after this, Xavier gets pissed and completely mind-wipes Erik, something he promised he’d never do. This causes a part of his psyche to splinter off, eventually kidnap Franklin Richards, and merge with him to become Onslaught.
Then an amnesiac, de-aged Magneto shows up to help the X-Men fight Onslaught.