Asteroid M is a legendary event in Marvel Strike Force that gives players the opportunity to unlock Magneto. Magneto is a legendary character, and the leader of the Brotherhood of Mutants. Once upon a time, Magneto and the Brotherhood used to rule the Arena. Times have changed, members of the Brotherhood of changed, but Magneto is still a great character. He still has that amazing ultimate Magnetic Vortex that groups together the other team and places blind on them. His passive, Brotherhood of Mutants buffs the entire team.
The Asteroid M event comes around every few months. It requires five mutant characters with at least five gold stars. Magneto is definitely worth the time, and I recommend unlocking him as the Brotherhood is a great War offense team who can counter several teams on defense such as S.H.I.E.L.D.