Complex sentence: In the third season, Damon assists Elena in bringing his brother, Stefan, back to Mystic Falls after Stefan becomes Klaus' henchman due to a bargain for his blood that would cure Damon of the werewolf bite he had received from Tyler.
Split into simpler sentences: In the third season, Damon assists Elena in bringing his brother, Stefan, back to Mystic Falls after Stefan becomes Klaus' henchman. The arrangement transpired after a bargain for his blood that would cure Damon of the werewolf bite he had received from Tyler.