Fey of the Fellnight Realm
Rhoswen is a powerful fey sorceress who was imprisoned within the Fellnight Realm millennia ago. Although the Fellnight Realm itself was not created specifically for her, but rather another powerful fey who had defied the Elder Fey, Rhoswen was also sealed away within the realm as punishment. The Fellnight Realm was designed to be an impregnable prison, but Rhoswen was somehow able to craft a staff that enabled her to break the wards on the demiplane, thereby allowing her followers to cross over into the material plane. When she and her minions attacked the Barony of Pohjola, the party crossed over into the realm and defeated her there, eventually resealing the wards and taking her staff with them to lock her away . . . hopefully for good.