Complex sentence: Kenshiro seeks out the highest-ranking and most powerful of the three overlords, Kaioh, Raoh's biological older brother, who plans a conquest to rule the post apocalyptic world in the name of evil by wiping out the followers of "Hokuto Shinken".
Split into simpler sentences: Kenshiro seeks out the highest-ranking and most powerful of the three overlords, Kaioh, who is in fact Raoh's biological older brother. Kaioh plans a conquest to rule the post apocalyptic world in the name of evil by wiping out the followers of "Hokuto Shinken".