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.

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

Connections
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 is exactly what Nemesis does, as revealed in Cold Days''.

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—later named Nemesis—corrupted Harry's first three big cases—Lily also spoke sadly of having watched her friend, Aurora, who she loved like a sister, change, becoming twisted. Then Maeve told Harry that Mab is infected. She implied that Lea 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 flamed. 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.

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. It seems Maeve was not curable in the same way, leaving Mab with no option other than to send Harry after her.