Uncanny X-Men (1963 1st Series) #111
"Mindgames!" Part 1 of 3. Story by Chris Claremont. Art by John Byrne and Terry Austin.
The bouncing -blue-Beast takes time off from the Avengers to attend the Circus. What he finds shocks him to his very core...the main attractions are none other than the X-Men! How did Xavier's team of heroes wind up in this position and who is responsible. This all sets up a last page that will make your blood run cold.
NOTE: 1st appearance of Mesmero.