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The people of Agartha once coexisted with the Nabateans, the children of the Goddess, but eventually turned away from the Goddess's protection and dedicated themselves to the advancement of technology for their prosperity. In time, they became greedy, arrogant, and imperialistic, eventually challenging the Goddess herself for dominance. The Goddess's daughter Seiros warred with the Agarthans and destroyed their nation for their hubris, but Fódlan was so devastated by the conflict that the Goddess was forced to expend most of her energy to heal it, and then fell into a long slumber in her Holy Tomb at Zanado. The surviving Agarthans retreated underground and their descendants became Those Who Slither in the Dark, who would devote centuries to plotting their revenge.
At some point, Those Who Slither in the Dark secretly manipulated Nemesis into breaching the Holy Tomb and killing the progenitor god in order to forge weapons such as the Sword of the Creator and create the Crest of Flames from her body. Nemesis then slaughtered the Nabateans at Zanado, and bestowed Crests upon his strongest warriors, the Ten Elites. Only Seiros and at least two other Nabateans, Cichol and his daughter Cethleann, survived the extermination of their people. Seiros persuaded one of Nemesis's allies, Wilhelm Paul Hresvelg into betraying Nemesis and used her own blood to give him the Crest of Seiros. The War of Heroes followed, a decades-long conflict between Wilhelm's Adrestian Empire and the forces of Nemesis, the King of Liberation. Seiros ultimately prevailed and slew Nemesis in Imperial Year 91, but the failure of Nemesis did not deter the Agarthans from their path. Over a thousand years later, Those Who Slither in the Dark prepared to inflict their vengeance against the unsuspecting people of Fódlan.
- In Shambala, words written in Cyrillic can be seen throughout the city.
- Many of the Agarthans are named after the Seven Sages of Greece; Thales, Solon, Chilon, Bias, Pittacus, and Myson.