The Glenmoril Witches are unique Hagravens encountered during The Companions quest "Blood's Honor". The quest is given to the Dragonborn by Kodlak Whitemane. The Glenmoril Witches can be found in a cave near Bilegulch Mine in the southwest corner of Skyrim. Named Glenmoril Coven.
There are five Glenmoril Witches in total. The Dragonborn is only required to slay one and retrieve her head in order to complete the quest. Killing more Glenmoril Witches allows the Dragonborn the opportunity to cure his or her Lycanthropy.
During combat, they taunt the player with phrases connected to the quest "Blood's Honor":
"The Beast has returned"
"You belong to Hircine now"
"Your blood is ours, Companion!"
The heads hold no value, besides being able to cure Lycanthropy. They are, however, worth four points of weight.