What is the demonic blade in Japanese?

Could you please clarify what exactly you mean by "demonic blade" in the context of Japanese culture or folklore? Is it a specific mythical weapon, a reference to a story or legend, or perhaps a colloquial term for a particular type of sword? If it's a reference to a specific item or concept, could you provide any additional details or context that might help narrow down the answer? Understanding the full extent of your query will allow me to provide a more accurate and informative response.