The X-Men and the Avengers: Gamma Quest: A Marvel Omnibus

The X-Men are mutant protectors of a world that fears and hates them. The Avengers are Earth's Mightiest Heroes, the greatest super-team ever assembled. When the Scarlet Witch of the Avengers and Rogue of the X-Men both disappear under mysterious circumstances, each team's search leads them to more questions than answers. Desperate to recover their missing teammates, they must join forces to uncover the truth. But their efforts will bring them up against a foe with the deadliest power of all: to make them turn on each other!