Complex sentence: It is for this reason that she is sought by Pompey the Great's son, Sextus Pompeius: he wants her to use her magic so that he can learn what the outcome of the Battle of Pharsalus will be.
Split into simpler sentences: It is for this reason that she is sought by Pompey the Great's son, Sextus Pompeius. He wants her to use her magic so that he can learn what the outcome of the Battle of Pharsalus will be.