Finishes: Colors & Stains

In a strange town near the Himalayas, Xavier encounters an alien calling himself Lucifer, the advance scout for an invasion by his race, and foils his plans. In retaliation, Lucifer drops a huge stone block on Xavier, crippling his legs.

The story of how Xavier was originally paralyzed was first told in X-Men #20 where Professor X recounts his battle with an alien called Lucifer in the Himalayas. During the fight Lucifer managed to crush Xavier's legs with a boulder, forcing him to use a wheelchair.