Plot summary
Vandal and his grandson Arion rebuild Atlantis. The Lords of Order recruit Arion, giving him the power to create humans with magic, Homo Magi. Vandal has Klarion sink Atlantis so it may conquer beneath the oceans; killing Arion and creating Homo mrmanus (modern Atlanteans) in the process. In the present, Child reveals the Lords of Chaos sent her to replace Klarion and is backed by their combined power. The Phantom Stranger teleports Zatanna to witness the fight between Klarion and Child, while taking her students to recruit Etrigan the Demon for support.
Despite Etrigan joining the fight, Child overwhelms all participants, forcing Klarion to retreat. Zatanna decides to ask for assistance to defeat Child. Meanwhile, Em'ree helps M'gann with her grief, and Vandal talks to his son Nabu /Doctor Fate, warning him of Child. In a mid-credits scene, Vandal prepares to have the Light and everything of value to him evacuated to Warworld.
Credits
Adapted from Wikipedia contributors, List of Young Justice episodes. HTML formatting and reference markers removed.