X-Men 55 - 56
This is where it all really begins, the beginning of the end and a run of issues that are the best in the X-Mens original run, possibly some of the best ever.
55 sees the return of Roy Thomas as writer, with Don Heck and Werner Roth maintaining art duties. Cyclops and his brother are kidnapped by the Living Pharaoh and taken to Egypt, while the X-Men give chase and are overpowered Cyclops attempts to fight free, struggling as he is he is a prime target for the Pharaoh, but it is a sudden unexpected blast from Alex that fells the bad guy.
56 One of the greatest Batman artists of all time, Neil Adams arrives and takes over the art, in a tale that sees the Pharaoh absorbing all the power from Alex, converting him to the nigh unstoppable Living Monolith. The story enhanced by Adams' stupendous ends with the X-Men managing to overcome the Monolith, but his powers flow back into Alex, who starts screaming he can't control it, and begins to build towards an explosion.
57 - 59 My favourite story of the original X-Men run sees the return of the Sentinels. It seems that Bolivar Trask, the man who designed the Sentinels had a son, Larry. Not only did he inherit his father's fortune, but also the hatred the man had for mutants; a hatred that has been fuelled by his old man's death.
Lorna Dane is coming closer to accepting her powers, but is over powered when her apartment is torn apart by a Sentinel. The X-Men lose a highly unstable Alex in Egypt, and are forced to head home. Alex struggling with self pity and his uncontrollable power is suddenly taken out of the picture by well you can guess. The X-Men return home and go their separate ways, unaware that they are being observed.
What follows is a rollercoaster ride, comic book storytelling at it's best as the Sentinels run rampage, the X-Men are taken out one by one, Magnetos' followers are all overpowered, Magneto turns out to be a robot (so where is the real one?). Alex is given a suit that controls his powers and takes the name Havok, old faces return, Vanisher, Blob, Banshee all fodder for the giant robots. Trask is convinced by an old friend that what he is doing is wrong, only to have his fathers' creations turn on him when they believe him to be a mutant. It just does not stop. In the end as Cyclops faces death he talks to the Sentinels, who have come to conclusion that all life is mutation, therefore all life must be destroyed. Cyclops sees the warped logic that is part of the Sentinels actions and manages to get them to trace the cause of mutation backwards. Once they deduce that the source of all life and mutation is from the sun, they leave, travelling to the heart of the sun to attempt to destroy it. They don't.
The whole thing is epic, Thomas writing in a manner he never managed on his first run, all backed up by the stunning Adams art and it does not stop here.