Human females (as far as we know as we have not observed any male witches) that have become twisted and deformed through practicing a form of magic which requires self mutilation for the empowerment of their magic.
As Daniel has said many times in the past, and as the group learned recently, “witches like to throw things.” They will often keep collections of items they can use to toss at their intended victims or those seeking to harm them.
The witches seem to have some common abilities gained through their “study” telekinesis, illusions, the ability to command a personas actions, invisibility, and a form short distance teleportation are among the powers observed.