Ichiyouso (一つの要素; lit. One Element) is the harnessed and manifestation of raw life-force, and is the most ancient method of manipulating energy. Although its use is identical to its successor, chakra, ichiyouso is considerably different in that, instead of mixing the energy within the body with the spiritual energy gained through experience, inchiyouso literally materializes life-force itself. Ichiyouso is vastly stronger than chakra insomuch that the energy it harnesses is far more potent and unrefined. Ichiyouso is composed of two integral parts; Kouzennoki and Seimeiryoku.
The Arkōn Kanmon was the first samurai to naturally utilize ichiyouso. No other samurai has ever had the natural ability to use ichiyouso, although few samurai of old claimed so, and the safer method of molding chakra quickly replaced it.
The Yakusu (要因; lit. factors) are the two vital components of utilizing ichiyouso. The two parts of the Yakusu are Kouzennoki and Seimeiryoku.
The Kousennoki Yakusu (普遍的な生命力; Universal Life-Force Factor) is the first factor of harnessing ichiyouso. Kouzennoki is the cosmic source of life-force that animates all living things, and is virtually ubiquitous, infinite, and omnipotent. Suffice to say, Kouzennoki represents the factor of potence, intensity, constraint, and force. The method of utilizing Kouzennoki involves drawing upon the life-force surrounding the individual.
The Seimeiryoku Yakusu (活力生命力; Vitality Life-Force Factor) is the second factor of harnessing ichiyouso. Seimeiryoku is the internal life-force that is produced individually by every living thing. Although not as powerful as Kouzennoki by nature, Seimeiryoku is far more yielding and easier to manipulate. Seimeiryoku represents the factor of control, regulation, restraint, and discipline. In simplified terms, Seimeiryoku can be summarized as essentially utilizing the power within one's self.
As with all powers, ichiyouso has an extreme price. At the expense of great power, the utilization of ichiyouso consumes the user's very life-force, literally eating their own life away. Indeed, those use actively used ichiyouso would often die immediately, or would age rapidly and die soon after. Those with the natural ability to utilize ichiyouso are immune to these demanding effects. The only known individual to be immune to the adverse effects of ichiyouso was the first samurai, Arkōn Kanmon. In ancient samurai times, after Arkōn Kanmon's death, there were others that claimed to be able to safely utilize ichiyouso, although it was never confirmed, and likely a ruse to gain prominence.
The legacy of ichiyouso lived on, however. Eventually, by the Second Great Samurai War, the Land of Spirits discovered a way to temporarily utilize ichiyouso by inducing a condition known as the Archon State, named in honor of Arkōn Kanmon.