X-Men: The Hidden Years

The X-Men are stranded in the Savage Land, having been sent back there to confirm Magneto's death. But now Marvel Girl's fate is in doubt. The tribe that subdued them claimed she had gone to the land of the dead, but Warren and Hank have learned that this is literally a different place. That tribe's old and infirm are transported somewhere else. Determined to find her, Scott sends Warren in by air, while he and Beast will try to follow the river that the old and infirm are sent down.

But the tribe views these activities as heretical, and they're determined to stop the X-Men.