The Nocnitsa, or "Night Hag", in Polish mythology, is a nightmare spirit that also goes by the name Krisky or Plaksy. The Nocnitsa is also present in Russian, Serbian and Slovakian folklore. She is known to torment children at night, and mothers in some regions will place a knife in their children's cradles or draw a circle around the cradles with a knife for protection. This is possibly based on the belief that supernatural beings cannot touch iron:

The Nocnitsa, or "Night Hag", in Polish mythology, is a nightmare spirit that also goes by the name Krisky or Plaksy. The Nocnitsa is also present in Russian, Serbian and Slovakian folklore. She is known to torment children at night, and mothers in some regions will place a knife in their children's cradles or draw a circle around the cradles with a knife for protection. This is possibly based on the belief that supernatural beings cannot touch iron