Complex sentence: Gambit was unable to prevent the Marauders from killing a considerable number of Morlocks, but he was able to save a single child named Sarah, who would grow up to be Marrow, the leader of the mutant terrorist group Gene Nation.
Split into simpler sentences: Gambit was unable to prevent the Marauders from killing a considerable number of Morlocks, but he was able to save one, a child. Her name was Sarah, and she would grow up to be Marrow, the leader of the mutant terrorist group Gene Nation.