Cornelia

After she ended the epidemic, one day Cornelia had a sudden shift of personality. Whether this is her true personality and she is from Agartha, or she was killed and replaced with an agent of Those Who Slither in the Dark is left ambiguous, but since that day she is only loyal to Those Who Slither in the Dark and doesn't care for anyone else. Despite people noticing this change, she was still trusted and the king appointed her a high post as a mage in service of the royal family.

When the second Queen Consort Patricia's daughter, Edelgard, was taken away by Those Who Slither in the Dark, the desperate Patricia turned to Cornelia and said she wished to see her daughter again, no matter who or what she had to sacrifice to do so. Cornelia claimed that the one truly keeping Edelgard away from her was King Lambert, and then said she could make Patricia's wish come true at the cost of the king's head, henceforth starting the Tragedy of Duscur.

Cornelia also used a young girl named Hapi in magical experiments related to the girl's mysterious Crest of Timotheos and its abilities. She had befriended the once-street urchin in order to get her trust before completely shattering it via first offering her good housing and food, then making her a guinea pig. The results of Cornelia's experiments resulted in monsters being drawn to Hapi whenever she sighed. Patricia was indignant towards Cornelia's treatment of Hapi and supposedly considered exposing her experiments, before she disappeared during the Tragedy of Duscur.

Following a failed attack by the Church to reclaim Garreg Mach, Edelgard decides to strike Arianrhod, as the city was full of mages, Cornelia included, and had special magical weapons that she did not want to risk having to deal with if ignored. The attack was to be planned in secret, though she hoped that if they took Arianrhod, they would be able to increase pressure on the Kingdom.

Rodrigue informs Cornelia that an attack force had left Garreg Mach and were making their way to Arianrhod. She is initially surprised by this development, before realizing that she was the main target. She does not explain her reasoning to him however, and has several Titanus prepared in advance to crush the invaders, including the Kingdom soldiers sent to protect her if given the chance. Despite the protection of the Kingdom soldiers and the Titanus, Cornelia is mortally wounded in the ensuing battle. However, she simply laughs her death off, simply stating that Edelgard has been acting "accordingly" to their plans. (In the Japanese version, she is instead impressed that Edelgard had her and Those Who Slither In The Dark out-smarted.)

Displeased that Edelgard had caused the death of one of his subordinates, Volkhard confronts her after the battle and imparts her with a vague threat about avoiding Arianrhod's fate before leaving. Immediately after, Hubert brings news that Arianrhod was leveled by "javelins of light," which instantly killed a third of the Imperial troops stationed there. The two deduce that this was the trump card of Those Who Slither in the Dark.

If Dimitri was sided with, Cornelia has Grand Duke Rufus, the Kingdom's regent and Dimitri's uncle, assassinated. She ensures Dimitri is condemned to death for the crime, making it appear as though the prince had believed the rumors of Rufus being involved in the Tragedy of Duscur and then murdered him for revenge. With Dimitri out of the way, Cornelia takes control of the Kingdom with the support of the western nobles and then invites the army of the Adrestian Empire into Fhirdiad, effectively surrendering the Kingdom to the Empire. Supported by Imperial troops, Cornelia and her "Faerghus Dukedom" set about quashing the rebellious eastern nobles, such as House Fraldarius, over the next four years. She revealed to Gilbert that she sought nothing less than the complete destruction of Faerghus.

Dimitri, who escaped his execution, rallies the Kingdom's loyalists in 1185, eventually entering Fhirdiad. The people go into open revolt, though Cornelia does not make any attempt to suppress them, knowing that she needed every available soldier to fight the loyalists. She also deployed several Titanus as a precaution, but despite her efforts, is mortally wounded in the ensuing battle. In her dying breath, she reveals that Dimitri's step-mother, Queen Consort Patricia, was seemingly responsible for the Tragedy of Duscur. She stated that Patricia had wanted to see her daughter, Edelgard, again no matter the cost, and that had apparently resulted in the killing of King Lambert and the other Kingdom nobles in the Tragedy. Dimitri was shocked by this revelation, but with Cornelia dead, he had no way of determining if she had spoken the truth about his step-mother's involvement and turned his focus to fighting the Empire.

While Cornelia is never directly fought, she is mentioned to be the mastermind of a coup d'état to overthrow the royal family. After Edelgard's defeat and death, Cornelia abandons Faerghus altogether, disappearing to parts unknown.

Currently, Cornelia is a power-hungry woman who is more than willing to claim the throne for herself, even if it means murdering Dimitri's uncle and framing the former for her crimes. Furthermore, she is a self-serving tyrant, as she was willing to given her territory to the Empire.

However, it is strongly implied that the real Cornelia was killed and replaced by Those Who Slither in the Dark, as several people note how her personality and behavior changed quite suddenly and with another member of Those Who Slither in the Dark referring to her as 'Cleobulus' (Note: In the English version, Thales refers to Cleobulus as 'he', but in the Japanese version no conflicting gender is given). In regards to impersonations, this much is also said about Volkhard, Tomas and Monica), and at one point after the time skip, a merchant wonders why she has imposed ridiculous levels of taxation on the parts of Faerghus she rules over, pointing out that it makes no sense to drain a region so completely if they intend to keep making use of it as a conquered land. Dimitri furthers that argument in Three Hopes by pointing out to her that if Cornelia really wanted to destroy Faerghus all along, she simply had to fail in curing the plague, with her answering that it was "another lifetime completely".

She's also considerably pragmatic, as when Dimitri's return inspires uprisings among the Kingdom citizens, Cornelia refuses to send troops to suppress them not out of any sense of mercy to the people, but rather because she knows she has to confront Dimitri with every soldier she has and can't afford to send any elsewhere if she is to have any chance of winning.

In Three Hopes, her alternate name, Cleobulus, is the name of one of the Seven Sages of Greece, in keeping with members of Those Who Slither in the Dark having the names of Greek philosophers.