Complex sentence: In the dark alternate future shown in "The Dark Knight Strikes Again", Captain Marvel is now visibly aged, with receding white hair and glasses, and is being blackmailed by Lex Luthor into working for him or Luthor will kill the captured Mary Marvel.
Split into simpler sentences: In the dark alternate future shown in "The Dark Knight Strikes Again", Captain Marvel is visibly aged, with receding white hair and glasses. He is being blackmailed by Lex Luthor into working for him, to prevent Luthor from killing the captured Mary Marvel.