What is "ysatters-kajsa"

Ysätters-Kajsa was a wind- troll that people in the Swedish province of Närke used to believe in; probably the only one of her kind in Scandinavia. The Swedish writer Selma Lagerlöf immortalised Ysätters-Kajsa in the first part of Chapter 24 of her famous novel The Wonderful Adventures of Nils (1906–1907).

She wrote that in the Swedish province of Närke, in the old days, there lived a troll named Ysätters-Kajsa. She was named Kajsa because wind-trolls used to be called by that name. Her cognomen Ysätter came from the swamp Ysätter in Asker parish where she was born. She appeared to have lived in Asker parish, but she played jokes on people all over Närke and was unique to that region.