Complex sentence: Arthur, Mera, Superman, Wonder Woman, Green Lantern, and Cyborg arrive at Atlantis and learn of Queen Atlanna's death, the Atlantean people being under the impression that a surface dweller killed her.
Split into simpler sentences: Arthur, Mera, Superman, Wonder Woman, Green Lantern, and Cyborg arrive at Atlantis and learn of Queen Atlanna's death. The Atlantean people were under the mistaken impression that a surface dweller killed her.