Complex sentence: Hundreds of years ago, in order to protect his mother and younger sister, he bound himself in a Geass contract with an unidentified individual and his Geass ability allowed him to ascend to power.
Split into simpler sentences: Hundreds of years ago, in order to protect his mother and younger sister, he bound himself in a Geass contract with an unidentified individual. His Geass ability allowed him to ascend to power.