Complex sentence: Aquaman visits the archives of Atlantis, where he discovers how Superman descended into tyranny and that Atlantis has agreed to complete subjugation under Superman, he is then forced to fight his counterpart as well.
Split into simpler sentences: Aquaman visits the archives of Atlantis, where he discovers how Superman descended into tyranny and that Atlantis has agreed to complete subjugation under Superman. He is then forced to fight his counterpart as well.