In Greek mythology, Zeus is the "Father of Gods and Men," the king of gods, who rules the Olympians atop Mount Olympus. He is also the god of thunder and ruler of the sky.
Cronos, his father, had sired six children with Rhea, his wife: Poseidon, Hades, Demeter, Hestia, Hera and Zeus himself. But Cronos swallowed them all because of a prophecy claiming that he would be overthrown by his son, as he had overthrown his own father. Rhea, seeking revenge, hid Zeus at birth and gave Cronos a rock wrapped in cloth to swallow instead.
Once he reached adulthood, Zeus set to fight his father. He made Cronos vomit the rock and his brothers and sisters, then he freed the Titans, the Hecatonchires and the Cyclopes, which started a war known as the Titanomachy. Once victorious, Zeus casts his father and the other Titans to Tartarus, then divides the world between himself, Poseidon and Hades. Zeus got the sky, Poseidon the sea and Hades the underworld.
Zeus is one of the four new Deities in Megami Tensei II, and is extremely similar stat-wise to Lucifer's recruitable form. To obtain him, Megami Tensei II must be started from a completed Megami Tensei save. After killing Bael's frog, he can be created by fusing Leto, Artemis and Apollo, regardless of the protagonist's level.
Zeus is encountered at the end of the fifth Sphere in the Womb of Grief. The protagonist arrives to find Zeus in combat with Alex. Zeus overpowers her, and George forcibly enacts a shutdown of her Demonica for safety. This prompts the protagonist to either intervene or observe, upon which Zeus turns his attention to the protagonist.
Zeus's fight revolves around physical and electric attacks, so prepare accordingly. Adamant Sickle is a physical attack that also raises his attack twice. Keraunos does elec damage to a target and inflicts a severe defense debuff on them, dropping the defense by 4 stages. Divine Lightning is also an almighty attack that can inflict mute. When he hits half health, he can also apply Sol Canus, a field effect that purges all buffs and debuffs while also preventing further buffs and debuffs from being applied.
It pays well to keep buffs and debuffs in moderation - don't fully buff or debuff or he'll respond with Dekaja and Dekunda to erase your efforts. When Sol Canus happens, play defensively and wait it out, or if his health is low enough, try and burst him down with Co-Op attacks, especially if Blitzkrieg triggers to give your team the turn advantage.
Upon defeat, Zeus can be summoned through a Special fusion of Wild Hunt, Oberon, Anzu, and Throne once the protagonist reaches level 67. He leaves behind the Fifth Fruit and warns the protagonist of what his sister is capable of, alluding to the events that happen in the new endings. The player also finds the Armor Shard, which is used to make the Co-Op Enhance W Sub-App, allowing the protagonist to Co-Op with demons of any alignment.
"The omniscient and omnipotent god in Greek myth, he defeated the storm giants to become the king of the Olympian gods."
—Warrior Zeus, Persona Q compendium
"The omniscient and omnipotent god in Greek myth, he defeated his father, Cronus, to become the leader of the gods. He has many consorts, divine and human, who bore him children."
—Zeus, Persona Q compendium
Zeus is summoned by Elizabeth as the culmination of all her work. Upon summoning the persona, his power proves too great for her (Which is evident as Elizabeth's level in the fight is one level lower than Zeus' base level), forcing itself into her and using her body to test the Persona users, seeking for them to gain power of their own will without relying on brotherhood or being a mere lamb in the herd. Both forms of Zeus are used in the battle. After the fight he is overjoyed by the power and strength they show, inviting them to come to his realm one day where glory and battle awaits in a shining world beneath Yggdrasil.
This fight is only available in the post-game, after defeating the final boss, but before starting a new cycle. After defeating him, Margaret tells the myth of Zeus' and Chronos' relations to each other to either protagonist. Margaret also states that Chronos is a symbol of death, Zeus is a symbol of life, and that both teams are closely related to him. Due to the world they are currently in existing due to Chronos' power, Zeus, as a symbol of life, was unable to enter, which is why he had to rely on Elizabeth summoning him to enter the world that kept denying him. The orbs she had the protagonist gather were tools related to him that could bring him into the world made by an icon of death.
After obtaining the Blessed Loincloth from the request, Time for the Finale, Warrior Zeus can be created, the easiest way being a rank-up fusion, by fusing Michael, Satan and Metatron. Zeus can then be created through special fusion by fusing Warrior Zeus and Seth.
The Fearsome Shard obtained by using Zeus in a sacrifice spread allows Theodore to create the best armor in the game, armor which grants +2 to all stats and has 380 defense.