In Ikebukuro there are two living urban legend, The "Black Rider" and The "Slasher".
The "Black Rider" Celty Sturluson is a dullahan. And The "Slasher" Saika (currently in the possession of Sonohara Anri) is an entity that manifests itself as a cursed Japanese sword.
Celty is based on Irish mythological figure, so is Saika also based on some old folklore?