Danu Talis was the mythical home of the Elders. It sank 10,000 years ago when the ruling twins with auras of pure gold and silver fought on The Pyramid of the Sun. In the modern world, it is known as Atlantis.
The Great Elders ripped the island from the seafloor to serve as the capital of their empire, abandoning the Nameless City to the grip of the jungle. It can be assumed that the reason they did this was to create the Pyramid of the Sun, centering it on the Isle as the nexus of the entire earth Shadowrealm. It was explained to the twins that at the top of Pyramid, one could change the world.
According to legend, the Sun and the Moon, otherwise known as the ruling twins, fought on the Pyramid of the Sun, and the energy released shook Danu Talis down to its foundations. The island was ripped apart and returned to the sea floor from whence it came. The magic polluted the very air, ensuring that the Next Generation of Elders born after the Fall were as different from their Elder parents as the Elders were from the Great Elders before them, and the Great Elders from the Archons before them.
In reality, the twins of legend, Josh and Sophie Newman, having been sent back in time by Dr. John Dee, using the Four Swords of Power to create a leygate, and had just destroyed the last two Earthlords, releasing the energy that the inhabitants of the city thought meant the twins were fighting. Josh received a message on an emerald tablet from Abraham the Mage that told (cryptically) of how he had to destroy Danu Talis for the world of man to ever exist. Josh then combined the Swords while Sophie fled the city, helping its inhabitants to safety along the way. The Four Swords melded into the shape of a hook, and Josh Newman became Marethyu, the shadowy puppeteer of the entire series. Harnessing the energy of the earthquake, created by Abraham in his dying moments, that was racing through the city and into the Pyramid of the Sun, Marethyu ripped the Island of Danu Talis from the earth and plunged it back into the sea.