It was chronicled in Dark Reign: The List - X-men. He was part of the Cabal and Osborn's Dark X-men team. He defected though during the Dark Avengers/Uncanny crossover. Basically Namor helped the X-men set up Utopia and in return, they've given his people safe haven and helped him set up New Atlantis, which he rules beneath the island. Here's his bio from the Heroic Age - X-men one shot
Thats true. The Avengers has some heavy hitters but the X-men overall have more powerful members. Scott's Extinction Team could wipe the floor with alot of them. Ive always said it, but Illyana can take most of them out in one swoop. I noticed, she conveniently wasnt there for the confrontation. She could teleport that hellcarrier and all those Avengers into Limbo, the moon or 100 years in the future if she wanted to
thanks. It should work now