Prior to its final destruction, the city was ruled by Poseidon, and it was one of the Sister Realms, which it formed together with the Underworld and Elysium. They appeared to defer to the authority of Mount Olympos.
The Greek philosopher Pythagoras compared Atlantis' function to that of a library, containing all of the knowledge of the Isu; including their physiology, scientific advancements, historical knowledge, culture, and beliefs. 
During the Isu Era, Atlantis was created to be an indestructible repository of Isu knowledge where both members of the First Civilzation and humanity could co-exist. The city was originally ruled by the Isu Atlas but he was supplanted by his power-hungry father, the Trident King Poseidon, who gave Atlas and his nine brothers the subordinate position of Archon to manage the city's affairs.
Poseidon, as the Dikastes Basileus, the Grand Adjudicator, implemented a cycle system for Atlantis wherein the city would be destroyed and reconstituted afresh every seven years if it was judged to be imperfect, a process which could wipe individuals from existence. He also eventually outlawed the use of experimentation on the city's human residents.
According to one Atlantean myth, a matter which led Poseidon's loathed brother Zeus to visit the city and reprimand Poseidon in person enraged the Trident King so much that he destroyed the very cycle in which the meeting had occurred.
Around the year 422 BCE, the Gateway to the Lost City, which connected Atlantis to the surface world, was visited by the Spartan misthios Kassandra. There, she discovered her biological father, Pythagoras, along with the Staff of Hermes, which had kept him alive for a very long time. Together, they used the Atlantis artifacts to seal the entrance to the city.
A simulated version of the city was later visited by Kassandra to help her master the Staff.
- Assassin's Creed: Odyssey (first appearance)
- The Fate of Atlantis: Judgment of Atlantis (simulation only)
- Assassin's Creed: The Essential Guide (2nd edition)
- ↑ Assassin's Creed: The Essential Guide (2nd edition)
- ↑ Assassin's Creed: Odyssey – Bios of the Gods
- ↑ 3.0 3.1 3.2 Assassin's Creed: Odyssey – The Fate of Atlantis – Welcome Home
- ↑ 4.0 4.1 Assassin's Creed: Odyssey – The Fate of Atlantis: Judgment of Atlantis – Isu codex: A Tale of Brothers
- ↑ Assassin's Creed: Odyssey – A Family's Legacy
- ↑ Assassin's Creed: Odyssey – The Gates of Atlantis