Reverberations of the Past

Upon once again defeating Legion, the assembled X-Men learn that "he is not the Legion they should fear". Between mad cackling, a story is told of their evolving, genetic materials extending far into the future and threatening to become the dominant lifeform in the universe. That is, unless a super-powered group of aliens aren't intercepted during their time travel to eradicate the mutant gene in the name of good.

xmen vs Legion of superheroes

(only those pictured)
(Edit: For the sake of fairness, LoS get Mon-El)
Location: Eternals Homeworld on Titan (Saturn)
Winner by Death or KO (no BFR)