Onimaru Kunitsuna/Origin

Origin
Onimaru (鬼丸) is the name of a famous historical Japanese katana. It is regarded as a National Treasure and is also one of the "Five Swords under Heaven" (天下五剣). "Oni" means "devil", and "maru" which, while literally meaning "circle", is also a common naming convention as the suffix of Japanese ships and swords. The sword was made by the swordsmith Awataguchi Kunitsuna (粟田口国綱). As legend goes, Hojo Tokimasa (北条時政) was having dreams nightly in which he would be tortured by a demon. One night, an old man appeared in his dream, professing to be the sword of Kunitsuna. The sword had become tarnished and promised to rid Tokimasa of the demon if he would clean the sword. Tokimasa immediately cleaned the sword and stood it upright in his room, after which the sword, moving by itself, fell and sliced the neck of a demon that had been engraved on a fancily decorated pot. From this point on, Tokimasa no longer had dreams of the demon, and thus called the sword Onimaru.