Complex sentence: Galbatorix, a Rider who was denied a new dragon after his was killed, and another Rider named Morzan, murdered Shruikan's true Rider, whereupon Galbatorix twisted the young black dragon's mind through dark magic so that he would serve him.
Split into simpler sentences: Galbatorix, a rider who was denied a new dragon after his was killed, and another Rider named Morzan murdered Shruikan's true Rider. Then Galbatorix twisted the young black dragon's mind through dark magic so that he would serve him.