Kogarasu Maru

The Kogarasu Maru (小烏丸), or ""Little Crow Circle", is a unique Japanese tachi sword believed to have been created by legendary Japanese smith Amakuni during the 8th century AD.
Blade classification and history.
"Kissaki Moroha Zukuri" (鋒両刃造) blades like the Kogarasu Maru are sometimes referred to as "Kogarasu Zukuri" (小烏造), since the blade of the Kogarasu Maru is shaped this way and is well known for its distinctive sugata. The Kogarasu Maru is unique as a bridge between the old double-edged Japanese ken (based on the Chinese jian) and the traditional Japanese tachi and eventual katana.
The Kogarasu Maru was designed with a curved double-edged blade approximately 62.8 cm long. One edge of the blade is shaped in normal tachi fashion, but unlike the tachi, the tip is symmetrical and both edges of the blade are sharp, except for about 20 cm of the trailing or concave edge nearest the hilt, which is rounded. A single "koshi-hi" (腰樋) style groove runs from the tang to the transition point where the blade becomes double-edged, and is invariably accompanied by a "soe-hi" (添樋). The hardening process yielded a straight temper line ("sugaha hamon", 直刃刃文) on both sides of the blade.
The Kogarasu Maru is currently in the Japanese Imperial Collection. The tang of the Kogarasu Maru is not signed but the blade is believed to have been made during the either the early Heian period or late Nara period, by the swordsmith Amakuni, who is said to have created the first curved Japanese sword and is believed to have lived during this period. Two other "Kogarasu Zukuri" blades exist from this era, but many other blades of this type have been created throughout Japanese history.
Tsuneyoshi Murata, a General of the Imperial Japanese Army during the Meiji era, created the first Gunto, famously known as Murata-To which is a Kissaki Moroha Zukuri style double edged Katana.
In popular culture.
Though rare, Kissaki Moroha Zukuri style Katanas have also been featured in Japanese novels, mangas and animes.
Rumors.
The Kogarasu Maru is believed by some to have been a cut down naginata or yari blade that was fashioned into a tachi, as its shape is similar to that of some pole arms. Nevertheless, no evidence of shortening is found on any of the three extant examples from this time period.