Complex sentence: Shortly after the Blackrock was removed from Plastic Man, and found its way into the hands of Batman, who shortly afterwards decided he needed its powers to help him stop a (then) rampaging Superman.
Split into simpler sentences: Shortly after the Blackrock was removed from Plastic Man, and found it's way into the hands of Batman. Who shortly afterwards decided he needed it's powers to help him stop a (then) rampaging Superman.