Complex sentence: A series of impostors, known as the False Dmitriys, each claimed to be Feodor's long deceased younger brother claimed (and at times actually held) the title of Tsar.
Split into simpler sentences: A series of impostors, known as the False Dmitriys, each claimed to be Feodor's long deceased younger brother. These impostors claimed (and at times actually held) the title of Tsar.