Complex sentence: To fight the witches, a strange familiar of unknown origin known as Kyubey looks for adolescent girls with the potential to become to make a contract with, granting magical powers in exchange of making a single wish of hers come true.
Split into simpler sentences: To fight the witches, a strange familiar of unknown origin known as Kyubey looks for adolescent girls with the potential to become to make a contract with. Once a contract is performed, the girl is granted magical powers in exchange of making a wish of hers come true.