Complex sentence: Freed, Diablo offered a potion to the Thing that would turn him halfway into a human, and in exchange for Ben's services for a year, he would give him the other potion that would finalize the change.
Split into simpler sentences: Freed, Diablo offered a potion to the Thing that would turn him halfway into a human. In exchange for Ben's services for a year, he would give him the other potion that would finalize the change.