Nemesis

Nemesis is a secretive mind-altering influence from Outsiders that shares the Outsiders' goals, which appear to be inimical to humanity and reality as we know it. It was first mentioned to Harry in Cold Days. 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 appear to know of it, so even speaking its name is risky. Those beings who know of it call it The Adversary instead. Some or all of the evidence of the Black Council's existence appears to be the work of Nemesis.