These swords were said to have absorbed the blood of the demons they first slayed and thus gain the power of the demons.
They were introduced by Misuzu Kusakabe as the Kusakabe Five Treasures (草壁五宝 Kusakabe Go Hou) at first but later in the story, Misao Kusakabe revealed that there were originally seven swords, and the Kusakabe merely ignored the existence of the two in Misao's possession as they thought it was a stain in the clan's history. The Kusakabe Seven Swords (草壁七剣 Kusakabe Shichi Ken) are the seven strongest swords of the Kusakabe clan. Onikiri and Kumokiri, in possession of Misao
The 5 Kusakabe Treasures in possession of Misuzu