Witches

Strictly speaking, Witches aren't monsters. They're humans who can access, channel, and manipulate mystical energies to make changes in the physical world. A Witch can be a male or female, and there are three known types: Borrowers, Naturals, and Students. Borrowers receive their powers by making a deal with a demon--usually selling their souls. Naturals are born with the ability to perform magic, but they have to work to develop their talents. Students, as the name implies, learn magic from other Witches, or are self-taught.
Borrowers owe their powers to demonic sources, so they tend to be evil, but the other types of Witch--called White Witches--use their powers for good. The Men of Letters used magic when they thought it was necessary and so do some hunters.
Depending on their strength, talent and training, Witches can demonstrate almost godlike abilities. They have no special weaknesses--aside from those that any human possesses--but iron can disrupt their powers, and they're vulnerable to magic, such as Witch-killing spells. They're mortal, but if they're powerful enough, they can hold off aging and extend their lives, making them virtually immortal. More powerful and complex spells require more than one Witch to cast, and for this reason Witches often form cooperative groups called covens. Covens also provide for the exchange of knowledge, as well as protection for it's members.
Many hunters believe that Borrowers are the most dangerous of Witches. People who sell their own souls to gain power are capable of committing any evil act. They're already damned; what else do they have to lose?