The power of the crystal has sent shock waves throughout the universe. Even the Avengers and Fantastic Four on Earth are clueless and powerless. The crystal opens a "gate into Eternity". The X-Men find themselves unable to defeat the "guardian" of the gate.
On a hunch that the "guardian" is a robot, with Banshee's powers he is able to take it down, not without paying the price of some injury. A second guardian, more powerful than the first, appears.
Suddenly, there is a blinding light and everything living has been transported inside the crystal. The Phoenix feels the crystal calling out to her, but it is the crystal working its effect on them, unleashing their innermost fears and nightmares. The crystal continues to grow and destroy reality as it absorbs it.
After studying the crystal, Phoenix thinks she has a plan that will stop the crystal. Unsure of the sacrifice she might have to make, she calls out to Corsair to look out for Cyclops, “the man [she] loves”, if she were to perish. It is revealed that Corsair is Cyclops’ father.
Phoenix struggles to undo the effects of the crystal but she is not powerful enough. Xavier hears her cries and joined with the rest of the X-Men, channel enough power to her to fix reality. While saving the universe by summoning immense power, the dark side of the Phoenix is stirred up. The good Phoenix banishes its dark side. The strain causes her to collapse. Cyclops is there to catch her, and together, the X-Men return home.
(Classic X-Men #15 is a reprint of Uncanny X-Men (Vol 1) #108.)