The Mugenjin (無限刃, "Unlimited Blade") is the personal sword of the former Isshin Shishi hitokiri Shishio Makoto, and the last killing sword forged by the swordsmith Arai Shakku. This makes it the direct predecessor, and "brother" of the two Sakabato wielded by Himura Kenshin during his years as a wanderer.
It has several unique properties to allow it to kill without losing its edge or otherwise stagnating. Its self-sharpening edge is berthed to it by having a sizable portion of the blade chipped away in a regular, serrated pattern, which allows it to lose none of its feel regardless of the number of men it slays. In addition, while most swords would rust and dull from continued exposure to human blood and oils, the Mugenjin appears to suffer none of these consequences, and instead the fat it has absorbed during the many years it was used allowed it to ignite itself and catch fire when friction was applied. This gave Shishio the ability to use fire-oriented techniques such as the Homura Dama and Kaguzuchi, as well as use gunpowder to incapacitate his enemies at point-blank by lighting his gloves on fire.
As with Shakku's later blades, it is extremely durable despite its thinness, being able to withstand a direct hit from the first strike of the Amakakeru Ryu no Hirameki when Shishio blocked it. However, it is not indestructible, as when its owner finally spontaneously combusted, it burned away with him.
In Shin Kyoto Hen, the sword finds its way jammed into Kenshin's right shoulder, and extracted by Arai Seikū to be the new holy sword of the Hakusan Shrine.
- This sword most likely symbolizes Shishio's belief of the weak being food for the strong as the sword gets stronger with each human it kills.