Sabretooth: The Adversary by Victor LaValle

The island nation of Krakoa has ushered in a bright new era for mutantkind - paradise after years of persecution.

But even mutants must deal with monsters in their midst, and Victor Creed is perhaps the worst. One of the first acts of the Krakoan Quiet Council was to exile the savage Sabretooth to the pit beneath Krakoa, locked away in an endless darkness for his countless crimes against both mutants and humans.

Now, you're about to find out what Sabretooth has been up to since he was banished... and it's not what you expect!