Complex sentence: Prior to the events of the series, mankind waged a battle of survival against the shadowy creatures of the Grimm until they discovered the power of Dust, which allowed them to fight back against the monsters.
Split into simpler sentences: Prior to the events of the series, mankind waged a battle of survival against the shadowy creatures of the Grimm. Then mankind discovered the power of Dust, which allowed them to fight back against the monsters.