Those Who Slither in the Dark

History
Those Who Slither in the Dark's first attempt at revenge occurred sometime after the Goddess had entered a long slumber due to the exhaustion of using her power to heal the war-torn Fódlan. While the Goddess slept at Zanado, guarded by her children, Fódlan was invaded by a foreign army. A bandit named Nemesis sought to liberate his people from the invaders, and was persuaded by agents of Those Who Slither in the Dark that he could attain the strength to do so by taking it from the Goddess.

Nemesis and his allies, the Ten Elites, killed the Goddess and most of her children, and Those Who Slither in the Dark fashioned weapons from their corpses and the Crests from their blood. Nemesis banished the invaders and was crowned the King of Liberation, but he became corrupt, and the atrocity he and his allies had committed to gain their victory would come back to haunt them. Seiros, the last daughter of the Goddess and one of the few survivors of the massacre at Zanado, lured one of Nemesis' allies, Wilhelm Paul Hresvelg, to her side and used her blood to imbue him with the Crest of Seiros.

Wilhelm founded the Adrestian Empire and, together with Seiros and the Four Saints, warred against Nemesis and the Ten Elites, eventually overthrowing them and conquering Fódlan. As Seiros used the Church of Seiros to revise history for her benefit and secretly prevented the people of Fódlan from ever again developing advanced technology or threatening her, Those Who Slither in the Dark turned to other plans. Over the following centuries, they conducted extensive experiments on the Crests and created the Crest Stones. They also manipulated Fódlan's leaders from the shadows, promising power to them and using them to subtly undermine the Church as well as sow chaos.

Over a thousand years after the defeat of Nemesis, the final stage of their revenge against the surface neared its fruition. They gained lofty connections among the nobles of the Adrestian Empire at a time when a rift had formed in its relations with the Church, and after the Insurrection of the Seven in Imperial Year 1171, they experimented on the children of Emperor Ionius IX to create an ideal ruler whom they could control. These experiments were previously conducted on children from House Ordelia, but with less success as the only survivor of the experiment had their lifespan dramatically shortened. Meanwhile, Fódlan was beset by tragedies and uprisings, weakening the Church, the Holy Kingdom of Faerghus, and the Leicester Alliance. Finally, a librarian of Garreg Mach Monastery, Tomas, was assassinated during his retirement and is taken as a disguise for Solon. Under the name of Tomas, he returns to Garreg Mach and resumes his role as the librarian to infiltrate the Church of Seiros.

Academy Phase
Those Who Slither in the Dark make their first move against the Church of Seiros by implanting a a dark mage during the Goddess's Rite of Rebirth in Imperal Year 1180. They make themselves an ally with the Western Church and the Death Knight and attempt to steal the remains of Seiros. Though they were able to open the seal put upon the coffin, they instead find the Sword of the Creator and shortly afterwards are slain by newly appointed professor Byleth. A couple of days later, the mysterious Flame Emperor reports to an unidentified lord that not only was the bones missing and the Sword of the Creator was instead found, but Byleth was able to awaken the sword's power despite its missing Crest Stone. The Flame Emperor suggests that they let the professor keep the sword as she does not have enough information to act. The lord is then given use of the Death Knight once more and the Flame Emperor leaves.

While Byleth and her students deal with some bandits, the Death Knight is told to kidnap Flayn, sister of the second-in-command of the church Seteth, and harvest her blood for upcoming experiements. However, when Byleth returns from the Kingdom, she is immediately given her mission of the month to find Flayn, and discover that the Death Knight is keeping her in the catacombs of Garreg Mach Monastrey and engage in combat. After the battle against the Flame Emperor Army, the Flame Emperor themselves meet with the professor for the first time, warning her over what will come and leaves. Byleth and her students then save Flayn, and another student called Monica, who went missing the prior year before she could graduate.

War Phase
though Edelgard joins forces with them, she makes it clear that she has every intention of defeating them if they continue to be a threat. After Edelgard captures Arianrhod, killing Those Who Slither in the Dark's agent, Cornelia, in the process, Thales threatens Edelgard by destroying Arianrhod with "javelins of light." After reuniting Fódlan under the rule of the Adrestian Empire, Edelgard uses her newfound powers to engage in an unseen and silent war against those who slither in the dark, eventually emerging victorious and wiping them out completely, ensuring peace for Fódlan.

Members

 * Thales (Leader)
 * Solon
 * Kronya


 * Cornelia
 * Myson
 * Odesse
 * Bias
 * Chilon