Hagatha

The witch who resides in Kolyma who imprisoned Valanice in the Quartz Tower, due to jealousy. She is the sister of Mordack and Manannan.

She is said to be the most powerful and evil of witches, and has a taste for human flesh. The skulls of her victims decorate the outside and inside of her cave.

King Graham encounters but is never able to defeat her, and for the most part just avoids her. Not only had she kidnapped Valanice, but she had also kidnapped the Antique Dealer's Nightingale.

Hagatha in Unofficial Games
In KQ2 (AGDI) she is said to be the sister of Manannan and Mordack as well. However this is strongly implied to be a connection to the Black Cloak Society, not a family relationship. Some of the hints to her belonging to the society includes the black cloak she keeps in her cave, as well as her mentioning that black is the dress code for members of their little community.

Graham defeats her by knocking her out of the Quartz Tower. However her body is never found, hinting she may have survived.

In KQ2 1/4, she tries to capture Valanice again, if Valanice accidently swims to Kolyma.

Real World
In the official King's Quest Collection trivia program, and King's Quest Companion, she is said to be sister of Mordack and Manannan. Though this fact is not brought up in any of the manuals or the official adventure games themselves, other than the trivia program.

Its also apparently an idea that Roberta had nothing to do with though Peter Spear and other King's Quest designers used it in the trivia game and companion;

"'I asked Roberta [...], and sorry to admit that she said she didn't remember the Black Cloak Society. It sounds cool. Perhaps she should have taken it more seriously and featured it in other games. With respect to Hagatha [being related to Manannan and Mordack]: Roberta's answer was, 'Would you like them to be related?' Then she said, 'Nope, Hagatha was her own seperate person.'" - Ken Williams at SierraGamers (6-9-2003) "...one final thing: Hagatha was never part in any discussion of a Black Cloak Society." - Roberta Williams at SierraGamers (6-9-2003)