Complex sentence: The event is designed to give thanks for the survival of Native American peoples following the genocide after the arrival of Europeans in the Western Hemisphere and to serve as a counterpoint to the traditional Thanksgiving story in which the Pilgrims shared a meal with Native Americans.
Split into simpler sentences: The event is designed to give thanks for the survival of Native American peoples following the settlement of Europeans in the Western Hemisphere. Organizers want it to serve in contrast to the traditional Thanksgiving story in which the Pilgrims shared a meal with Native Americans.