X took over city/region/country/whatever, group of surviving heroes, vigilantes and villains who aren't in X's group are working together to fight against X. B villain (which is surprisingly glue to the team and a great asset/leader) manages to take out X, at the cost of being hit by a weapon of X's and getting killed by it,,,or is it? Turns out, X's weapon is an age-regressor, and now B went from (insert adult age) to (insert teenage/child age) with few/no memories of their life after the point they regressed to.