X-Men #20-#21: The Return of Lucifer

X-Men #20 - the first to be written by Roy Thomas - further plays with the problem of secret identities by having the Blob and Unus (who have teamed up after meeting at a circus) rob a bank while dressed as X-Men. The X-Men's attempt at distinguishing the real X-Men from the impostors is not entirely succesful. Unus and Blob are being manipulated behind the scenes by Lucifer, who has recovered from his apperance in issue #11, and we are now shown in flashback how he caused Xavier's paraplegia. Lucifer has a larger plan - he wants to subjugate the Earth's population using a machine, Dominus. After his defeat by the X-Men he is exiled to the so-called "Nameless Dimension" by his bosses.

Meanwhile, Cyclops - perhaps inspired by his encounter with Mimic's machine in the previous issue - briefly tries to leave the X-Men to seek out a doctor to help him control his optic blasts. At this point his powers are actively dangerous - #11 had him be unable to control his optic blasts when his glasses are removed - he couldn't even close his eyes to stop it. This plot thread gets forgotten by the end of the story, with Cyclops forced by events to team up with the rest of the X-Men.

Continuity

There's a new Cerebro after Magneto destroyed the first one in #18. Lucifer is identified as an alien, using the name "Agent One" with his people. New jet appears, apparently having been purchased before the Sentinel arc but not used yet.