Those Who Slither in the Dark

Those who Slithers in the Dark is the main antagonistic faction in Fire Emblem: Three Houses. They are a mysterious terrorism faction led by Thales and are at war with the Church of Seiros. The Flame Emperor and their army allied with them to achieve its goals, but those goals are seemingly at odds with Thales' own.

They were responsible for the Tragedy of Duscur and the following mayhem that stirred the whole game' plot.

Past
Those who Slithers in the Dark are descendants of the long lost Argatha Civilization in Fódlan. The people of Agartha once coexisted with the Nabateans, the children of the Goddess Sothis, but eventually they turned their back on the Goddess and dedicated themselves to the advancement of technology for their prosperity.In time, the Argarthans became greedy, arrogant and imperialistic, eventually challenging Sothis herself for dominance.

To punish the Argarthans' hubris and arrogance, the Goddess's daughter, Seiros, fought a war with the Agarthans and destroyed their nation for their hubris. However, Fódlan was so devastated by the conflict that Sothis was forced to expend most of her energy to recover it, causing she to fall into a long slumber in her Holy Tomb at Zanado.

However, this didn't mark the end between Argathans and Nabateans. The surviving Agarthans retreated underground to what would be their base, Shambala, and their descendants became Those Who Slither in the Dark, who would devote centuries to plotting their revenge.

Flame Emperor Army (dissolved)

 * Flame Emperor (deceased in all routes except Crimson Flower chapter)
 * Death Knight (deceased in all routes except Crimson Flower chapter)
 * Metodey (deceased)
 * Hubert von Vestra (deceased in all routes except Crimson Flower chapter)
 * Kostas (deceased)

Nemesis & Ten Elites (All deceased)

 * Nemesis (leader)
 * Blaiddyd
 * Riegan
 * Lamine
 * Goneril
 * Charon
 * Fraldarius
 * Gloucester
 * Dominic
 * Gautier
 * Daphnel
 * Maurice (formerly the Eleventh Elite; casted out after his transformation into a Beast)

Trivia

 * None of the four routes in Three Houses had fully revealed the overall plan of Those Who Slither in the Dark nor their true intentions.
 * The Blue Lion Route/Azure Moon chapter is the only route where Those who Sliter in the Dark doesn't get eliminated, even with the death of Lord Arundel, who was revealed to be Thales. Dimitri deduced there were people plotting behind the Tragedy of Duscar in secret, but he had not aware of their full existence or discover Shambala.
 * In Black Eagle route/Crimson Flower chapter, the elimination of the organization went offscreen, and the protagonists are actually in league begrudgingly with them, although both Hubert and Edelgard knew how far the organization shall go and would eliminate them once they had done with Rhea.
 * Barring Edelgard (Flame Emperor), the Death Knight and Hubert, Kronya was the only playable character in Fire Emblem Heroes that came from a member (not ally) of Those who Slithers in the Dark. However, Edelgard's Flame Emperor form is currently not available in Fire Emblem Heroes.
 * Only the Golden Deer route/Verdant Wind chapter had fully uncovered the deepest secrets lied within the organization's overall plan, with the reveal of Nemesis and the Ten Elites' true nature as villains. The Church of Seiros/Silver Moon chapter