Thales

Thales is the leader of Those who Slither in the Dark and one of the two main antagonists in Fire Emblem: Three Houses (alongside Seiros), being Solon and Kronya's leader as well as the employer of the Flame Emperor.

He is the main antagonist in the first half of the game, also known as the Acadamy Phase. Post-timeskip, depending on player's choice, he becomes a supporting character in the Black Eagle route (as Lord Arundel), a major antagonist in Blue Lion route (as Lord Arundel), the main antagonist in the Church of Seiros route (as Thales) and the secondary antagonist in Golden Deer route (as Thales).

He is voiced by Masaki Terasoma in Japanese version, and by Christopher Corey Smith (credicted as Chris Smith) in English version with the latter also voiced Slayde in Fire Emblem Echoes: Shadows of Valentia, and Legion and Reinhardt in Fire Emblem Heroes.

Overview
Thales is a priest of Agartha and one of Those Who Slither in the Dark. In order to carry out his master plan, he acts in the shadows of Fódlan under the disguise of Lord Volkhard von Arundel, uncle of Edelgard von Hresvelg and step-uncle of Dimitri Alexandre Blaiddyd. He is also responsible for the tragedy in Edelgard's family by the Crest experiment on her and her sister.

Edelgard, in her route, later reveals that her uncle Volkhard, Patricia's brother, was the mastermind behind the Tragedy of Duscur. Volkhard and Thales are one and the same, and the Tragedy of Duscur was used to destabilize Faerghus and make it easier for Cornelia Arnim to gain a foothold in the Kingdom's affairs.

Acadamy Route
Thales first appears in the Acadamy Phase as soon as the plot twist happened when Jeralt is killed by Kronya, disguised as Monica. Trying to save their father, Byleth to use Divine Pulse to rewind time. However, just as they draw the Sword of the Creator to intercept Kronya, Thales teleports in and blocks the attack, leaving Jeralt's assassination an inescapable fate.

After Jeralt's death, Thales and Kronya met the Flame Emperor, who was clearly disgusted by their crimes in spite of being their working partner.

Trivia

 * It is unclear if there was an original Volkhard whom Thales replaced just as Solon replaced Tomas and Kronya replaced Monica von Ochs, or if the identity of Volkhard was fabricated.