The Dark Court is the opposite of the High Court. It is thought to have been one of the two original courts which resided in Faerie; the Dark Court left Faerie to live among humans, and the High Court stayed. It is the court of the darker emotions and realities; the court of temptation.
Dark Court fey feed off what Irial calls the "finer" emotions in other fey: pain, fear, lust, jealousy and rage. As such, the Dark Court thrives during periods of turmoil among the fey and weakens during periods of peace. The Dark Court is starving and weakening because of the new peace between the courts of Summer and Winter. They weaken enough that one of their fey is killed by a human bullet-- something that shouldn't have been possible.
The Monarch of the Dark Court is always chosen among the solitary fey, to give the court balance. Without this, the court would self-destruct. Once a Monarch retires and hands over the throne to someone else, he or she becomes solitary again; Irial, who is the Dark King during Wicked Lovely and Ink Exchange, is the sole exception because he wants to stay and help Niall. He is the only previous monarch that is to be known anything about, though Devlin hints in the fourth book that there have been several Dark Kings and Queens before Irial. Like Irial and Niall, all of the previous Dark Kings embodied temptation (whether or not this also applies to the female rulers is not stated). Currently, the court is being ruled by Niall.