Shattered Crystal

The destruction of the Sixth Cosmos unfolded as follows:

On a remote nameless world there existed the M’Kraan Crystal, a mysterious gem which housed within it enough power to destroy all of existence.

The Dweller in Darkness, an extra-dimensional demon who fed on the fear of others, had recently been awakened from his centuries-long slumber...

... and now plotted to destroy the crystal so that he may feast on the waves of terror that would accompany the universe’s eradication.

The crystal was gravely damaged, and the only hope for saving it—and thus the universe—was to summon the Phoenix Force, the crystal’s legendary guardian.

Despite a valiant effort the Phoenix was unable to heal the crystal and prevent its shattering.

But, even as the universe entered its death throes, the Phoenix found a way to thwart the Dweller’s intentions. 

Using her immense telepathic powers, she sent out a message of hope to the doomed denizens of the universe—instructing them not to fear their impending deaths but rather to take comfort in the knowledge that something far greater would evolve from it.

His scheme thwarted, the despairing Dweller was consumed by his own minion.

bibliography:

Most of our knowledge of the Sixth Cosmos comes from the following series: X-Men Adventures (3 volumes, 1992–1996), Spider-Man Adventures (1994–1996), The Adventures of Spider-Man (1996–1997), and The Adventures of the X-Men (1996–1997).

The M’Kraan Crystal of this reality first appeared in X-Men Adventures #4–7 (1995).

The Dweller in Darkness began to awaken in The Adventures of the X-Men #4 (1996).

The tampering with the crystal was first detected in The Adventures of the X-Men #11 (1997)

The Phoenix’s doomed attempt to save the crystal took place in The Adventures of the X-Men #12 (1997).