Complex sentence: The Manhunters were discovered by the Justice League who seemingly defeated The Grandmaster Manhunter, and one of their human pawns, Mark Shaw, adopted a new identity as the costumed hero, The Privateer.
Split into simpler sentences: The Manhunters were discovered by the Justice League who seemingly defeated the Manhunter's leader the Grandmaster. One of their human pawns, Mark Shaw, adopted a new identity as the costumed hero, the Privateer.