From Discworld & Terry Pratchett Wiki
"Black Aliss" Demurrage is at once the envy of most Witches and a cautionary example. She may have been the most powerful of them all : she once sent a entire castle to sleep, unchanged, for a hundred years. Aliss's look and feel would become the standard for generations of Witches and the perception of the public. Her silhouette may still be seen flying across the moon on shop windows and schoolroom walls every autumn. She was known as "Black Aliss" long before her anti-social excesses; the nickname came from her fingernails. She came from the Forest of Skund, an intensely magical bit of the Ramtops neighbouring Lancre, and became famous across the continent for showy magic and some nasty fits of pique. Eventually, she went over the edge. She built the confectionery cottage. She cackled incessantly. Finally she was so out of touch with reality that two Überwaldean children could push her into her own oven. Now, powerful Witches covet the glory that was "Black Aliss", but they dread letting slip a cackle, and most feel their job is to guard against magic, not wield it.
Certain elements of Black Aliss resemble those of Black Annis, a figure in old English folklore.