Complex sentence: The player assumes the role of a Rider who befriends monsters by stealing eggs and hatching them, you then have the ability to name them, ride them in the field, and have them join you in battle.
Split into simpler sentences: The player assumes the role of a Rider who befriends monsters by stealing eggs and hatching them. You then have the ability to name them, ride them in the field, and have them join you in battle.