Nemesis

Nemesis, also known as The Adversary or the Contagion, is a secretive mind-altering influence from Outsiders. It was first mentioned to Harry in Cold Days.

Description
Nemesis is a kind of contagion that infects a person—humans or Supernatural beings—and changes their nature, makes them to act in ways not natural to them and causes them to do things harmful to others. It shares the Outsiders' goals, which appear to be inimical to humanity and reality as we know it. Those affected (or infected) by its influence have their natural desires warped into serving sinister purposes.

Nemesis keeps its existence secret by infecting or killing any who appears to know of it, so even speaking its name is risky. Those who know of it call it "The Adversary" instead. Infected creatures are able to act against their fundamental natures. For example, infected Sidhe become able to lie. Some or all of the evidence of the Black Council's existence appears to be the work of Nemesis.

It is also possible that Nicodemus Archleone mentioned it during the conversation with Harry in the Oceanarium. He said ''“Have you not seen the signs around you? Beings acting against their natures? Creatures behaving in ways that they should not? The old conventions and customs being cast aside?”''. which are the symptoms of Nemesis, as described in Cold Days. Later in the same conversation with Harry, Nicodemus refers to what's happening as a "contamination," which in turn is a synonym of the word "contagion."

Cold Days
In Cold Days, during a meeting at the Botanical Gardens Lily waves her hand over Harry supposedly testing for something in Harry. Lily implied that the Contagion corrupted Harry's first three big cases: Victor Sells, the Hexenwolves and the Nightmare. Lily also spoke sadly of having watched her friend, Aurora, who she loved like a sister, change, becoming twisted. Later, Maeve told Harry that Mab is infected. She implied that the Leanansidhe spread the contagion to Mab when she was bound in ice at Arctis Tor. They told Harry that he must kill Mab—as the Winter Knight, Harry is the only one who can. Maeve says Harry will act to save people, it's who he is.

Harry summoned Titania to ask her if Mab had changed or is still Mab because he had to know if Mab had been infected by the "Contagion" as Lily and Maeve claimed. At first she refused to help him. Then she whispered in Harry's ear that the name of the power he fears is "Nemesis" and to speak it carefully or it may hear him.

Later on, Harry realized that Cat Sith had been infected earlier and tried to help him. Cat Sith fought it for a moment, but lost and Nemesis spoke to Harry directly, pledging to kill him. As it attacked, Harry blasted it with magic and hurled Cat Sith into Lake Michigan.

Eventually, it is revealed that the Leanansidhe truly had become infected, via the Athame she received at Bianca's Ball. Mab was able to cure the infection, and remain unaffected herself, but not before the corruption had spread to Maeve. Mab implies that she can only cure those who want to be cured, but Maeve considers it freedom to be allowed to lie and serve her own desires instead of fulfilling her duties and rejects all help. With no other options, Mab ordered Harry to kill Maeve.

Known infectees

 * Victor Sells
 * Phillip Denton
 * Leonid Kravos
 * Vittorio Malvora
 * Leanansidhe
 * Aurora
 * Maeve
 * Cat Sith