Complex sentence: William Piper was a mortal that Gwen fell in love with, she subsequently moved to the mortal world and stopped using magic, which upset and possibly angered Aggie, and it is implied that by the time of the movies he has died.
Split into simpler sentences: William Piper was a mortal that Gwen fell in love with. She subsequently moved to the mortal world and stopped using magic, which upset and possibly angered Aggie, and it is implied that by the time of the movies he has died.