Complex sentence: Charlie reunites with Helen, who also convinces Rashid and Juri to form an alliance with them to retrieve the pieces, revealed to be the keys to control the Black Moons which were sent to certain individuals in order to prevent their use.
Split into simpler sentences: Charlie reunites with Helen, who also convinces Rashid and Juri to form an alliance with them to retrieve the pieces. She explains they are the keys to control the Black Moons, which were sent to certain individuals in order to prevent their use.