X-Men #104 (7.5)

1st appearance of Corsair, father of Cyclops, Havok and Vulcan

1st appearance of Ch'od, a member of the Starjammers

2nd cameo appearance of Lilandra

1st mention of Mutant X aka Proteus

1st meeting and battle of Wolverine and Magneto 

Cyclops pulls the team together after their first mind numbing encounter with Proteus. Proteus takes over the body of his father in order to take the X-Men down in round two.