The X-Men #65

The X-Men make it back to the mansion after their harrowing battle with Magneto only to be commanded inside by Alex and Lorna. They learn that Earth is about to be attacked by the alien Z'Nox, but a plan has been made to counter the invasion -- a plan formulated by none other than the \"dead\" Professor X. Xavier returns to his charges and explains that Changeling was masquerading as him when he was killed. The X-Men board the Z'Nox spaceship and attack, but it is the collective power of happy thoughts funneled through Xavier into his students that ultimately drives the aliens off.