Complex sentence: Meanwhile, the Black Rose Seminar appears in Ohtori Academy, led by "high-school" genius Mikage as a front for creating Duelists capable of defeating Utena so he may kill Anthy and replace her with Mamiya, a Rose Bride of his own choosing.
Split into simpler sentences: Meanwhile, the Black Rose Seminar appears in Ohtori Academy, led by "high-school" genius Mikage as a front for creating Duelists capable of defeating Utena. Instead of winning Anthy, he wants to kill replace her with Mamiya, a Rose Bride of his own choosing.