Thales

At some point, Thales assumed the guise of Volkhard von Arundel, head of a minor noble family within the Adrestian Empire. Volkhard was granted the title of lord after his sister, Anselma, became a consort of Emperor Ionius IX and gave birth to Edelgard, though shortly after she would be exiled from Enbarr. During the Insurrection of the Seven in Imperial Year 1171, in which Volkhard played a key role supporting Duke Ludwig von Aegir, Volkhard took his niece, to the Holy Kingdom of Faerghus in apparent exile. Anselma, now going by Patricia, had since married King Lambert, while Volkhard and Edelgard returned to the Empire in 1174.

A year before the events of the game, Thales sends his subordinate Solon to infiltrate Garreg Mach Monastery by posing as the librarian Tomas. Later on, Thales also sends Kronya, posing as the missing Officers Academy student Monica, to assist Solon.

Thales first appears as Volkhard in the aftermath of an attack on the Holy Mausoleum to convene with the Flame Emperor regarding the revelation of the Sword of the Creator, and requests the temporary use of the Death Knight's services.

When Jeralt, Byleth, and their class are tasked with investigating a ruined chapel, they find Kronya (still as Monica) in the ruins, who assassinates Jeralt. This forces Byleth to use Divine Pulse to rewind time and attempt to save their father. 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. Kronya is surprised to see Thales save her, though he says it is only because Kronya still has a purpose to fulfill. The two then teleport away from the scene as Byleth mourns their father.

A week later, Thales and Kronya are seen outside the monastery walls meeting up with the Flame Emperor. Thales scolds Kronya for her carelessness while praising the Flame Emperor as one of his greatest creations, and that by empowering them with the blood of "the defiled beast", the Flame Emperor will be the key to the salvation of his people. The Flame Emperor dismisses this, saying that neither he nor his people will receive the salvation they seek after his group's involvement in the Tragedy of Duscur and the Insurrection of the Seven. If Byleth is teaching the Blue Lions house, they and Dimitri overhear the conversation and learn that Thales and the Flame Emperor are somehow related to both incidents, spurring Dimitri's need for revenge. However, before the two can act, the Flame Emperor suspects someone is nearby and leaves the scene.

Near the end of the academic year, the Flame Emperor, now revealed to be Edelgard, ascends the Adrestian Empire throne as its new Emperor and declares war on the Church of Seiros, leading to a massive invasion. In all routes except Crimson Flower, the Knights of Seiros are able to resist the first wave, but a second appears more massive than before, forcing Rhea to transform into her true form. While she decimates the second wave initially, Demonic Beasts pile onto her, spurring Byleth to come to her aid. However, Thales appears and hurls a ball of magic, sending Byleth careening over a ledge into the valley below, not to be found by anyone for over five years.

Thales, remaining under the guise of Volkhard, continues to support Edelgard's efforts to take down the Church of Seiros, though Edelgard and Hubert barely mask their contempt for him. In the paralogue Darkness Beneath The Earth, he requests their aid in dealing with some of his "experiments" that have gone out of control in the Sealed Forest. Hubert hypothesizes that this is mostly an attempt by Those Who Slither in the Dark to assert their power over the Empire, but states that it has not deterred his efforts to investigate and eventually eliminate them.

When the Imperial army marches into western Faerghus, Edelgard besieges Arianrhod and kills Cornelia, knowing that she is one of Thales' subordinates. Thales, still disguised as Volkhard, asks Edelgard why she killed Cornelia without waiting for her to sabotage the Kingdom army from within, but his question is left unanswered. He then tells Edelgard that her rash decisions would negatively impact the future of the Empire, and says that it would be a shame if it were to turn into "another Arianrhod". In retaliation for Cornelia's assassination, he destroys Arianrhod with the "javelins of light", killing most of House Rowe's important members and a third of the Imperial troops stationed within in another attempt to keep her in line, though it had the opposite effect.

During the battle at the Tailtean Plains, Hubert briefly suspects Thales' involvement when a Kingdom soldier transforms into a Demonic Beast, but quickly dismisses the idea as Dedue states that he was the one who took several Lost Crest Stones from the Kingdom's castle and gave them to his soldiers.

On the eve of the final battle with the Knights of Seiros in Fhirdiad, Thales congratulates Edelgard on her victories thus far and tells her that the Empire is one step closer to ruling the world. However, as Edelgard has no intention of fulfilling his ambitions, she calmly retorts that such a future may not come to pass.

In the epilogue, Edelgard begins her silent war against Those Who Slither in the Dark. His fate is left ambiguous but it is highly likely that he was killed in the epilogue.

After Edelgard assumes the throne, Ludwig flees the Empire after being placed under house arrest. Thales, under the guise of Arundel, secretly takes control of Ludwig's lands, imposing even harsher taxes and enforcing conscription on their citizens, all doing so in Ludwig's name to ensure that all of the blame would be directed towards him.

He maintains his disguise and personally leads the invasion of the Leicester Alliance, but is ultimately defeated when Byleth and the newly restored Holy Kingdom of Faerghus arrive to reinforce the Alliance in response to Claude's summons. After his defeat, Thales denies Dimitri the satisfaction of learning about his involvement in the Duscur incident and encourages him to kill Edelgard, revealing to the rest of his forces that they are family. He rejoices in the "light" with his last breaths.

During the war, Thales learns that Byleth--and by extension Sothis--has reawakened and is leading the Resistance Army/Leicester Alliance Army against the Imperial forces. He attempts to kill their forces at Fort Merceus by using javelins of light to destroy the fort; however, this attempt fails as the Death Knight warns them of the incoming barrage and encourages them to flee after the battle.

Following the victory of the Resistance/Alliance Army against Edelgard at the Imperial Palace, they discover the location of Shambhala via a posthumous letter from Hubert, who discovered the source of the "javelins of light" after they were launched on Fort Merceus. Seeing that the last opportunity for the Agarthans to achieve their long-desired vengeance against the "children of the goddess" is at hand, Thales commands his remaining generals to use every available resource at their disposal, including the Titanus and Viskam cannons, to kill the invaders, even if it means destroying Shambhala in the process. Unwilling to accept defeat, Thales activates one last barrage of javelins of light with the intent of killing his enemies along with him. Rhea intercepts most of the javelins by transforming into the Immaculate One, but a few stray shots damage Shambhala and Thales is crushed to death by the falling rubble.

Thales first appears in his Volkhard guise when he learns that Edelgard is plotting a coup against him and Ludwig to reclaim the Imperial Palace. Edelgard, with the help of Counts Leopold Bergliez and Waldemar von Hevring, and some of her Black Eagles classmates, capture Ludwig and force Thales to drop his disguise and flee from Enbarr.

Almost three years later, Thales aligns himself with Ludwig and instigates a series of riots in Empire-controlled Hrym and Ordelia territories, forcing Edelgard to divert their attack on the Kingdom to deal with the internal chaos. Ludwig captures Fort Merceus and plans to use it as a springboard for an attack on Enbarr, but the Imperial army kills him, as well as Kronya, during their attempt to reclaim the fort. Thales lays low after this attack until he learns of Rhea and the Knights of Seiros advancing on Garreg Mach Monastery to reclaim it and kill Count Grégoire Varley, using the opportunity to foment further chaos by attacking Imperial and Church forces alike.

During the final battle for control of Garreg Mach, the Imperial army fights their way past the remaining Knights of Seiros and Those Who Slither in the Dark, facing Thales in the reception hall of the cathedral. Thales is initially defeated and retreats, but reappears while Shez, Edelgard, and the Imperial army are busy fighting Rhea. After both leaders are defeated, Thales attempts to charge a powerful spell to eliminate both of his enemies, but Shez throws their sword at his leg to pin him to prevent him from escaping, causing him to aim his spell instead at a charging Rhea. The resulting explosion causes both Thales and Rhea to disappear, with Edelgard assuming both killed. Thales' apparent demise causes Those who Slither in the Dark to vanish.

Thales allies with Ludwig, but cares less about his new allies, manipulating both him and Edelgard from the shadows as the former Empress is a shadow of herself. After Ludwig is slain by Kingdom forces, Thales retreats to Garreg Mach, using both Grégoire and the remaining Imperial forces to defend himself. In the ensuing siege, the Imperial forces are routed, leaving him with both Edelgard and a few troops under his command. After Shez knocks Edelgard out of range, Thales tries to defend himself from Dimitri by launching numerous orbs and a barrier, but he shrugs and reflects each one before breaking his barrier and impaling him with Areadbhar, killing him.