Complex sentence: When it comes to Colossus, it is revealed that Magik drove the Cytorrak's possession out of him, leaving Colossus deeply depressed, which leads to Magneto's decision to leave him alone.
Split into simpler sentences: When it comes to Colossus, it is revealed that Magik drove the Cytorrak's possession out of him. This leads to Magneto's decision to leave him alone.