Complex sentence: Rama forms an alliance with Sugriva, whom he will help to defeat Vali and regain his wife Ruma and his kingship; in return Sugriva will aid in the search for Sita.
Split into simpler sentences: Rama forms an alliance with Sugriva, whom he will help; in order to defeat Vali and regain his wife Ruma and his kingship. In return Sugriva will aid in the search for Sita.