Yin (in Japanese: 魄 Haku; Yin energy) is a dark energy that inhabits every soul and is materialized by sin and negative emotions. It plays a primary role in Shinobi (2002).
Centuries ago, the ancient demon blade, Akujiki, was forged. The blade is inhabited by an age-old and demonic entity that must consume Yin's darkness in order to survive and gain power. As the demon consumes Yin, its power grows exponentially, and the souls inside will form into an amalgamation of hatred and pain and sorrow. Many have fought and died to acquire its power, for it is said that he who wields Akujiki can rule over the souls of men.
Hiruko Ubusuna was a skilled onmyouji (in Japanese: 陰陽師； sorcerer or medium) who lusted after Akujiki's power. After he was first defeated by the Oboro clan, he laid dormant inside the Kumano Shrine for decades before finally being released. Upon his awakening, he sensed Akujiki's dormant power and devised a plan to awaken the blade. Four years prior, the Oboro clansmen Hotsuma and Moritsune were locked in a fight to the death. The two brothers were the eldest members of the clan's main household, and they were forced to duel for the clan's leadership role. This savage ritual was initially devised to suppress the demon blade's hunger by providing it a sacrifice that was filled with Yang, the positive light energy and spiritual opposite of Yin. The battle was swift, and Hotsuma prevailed as the victor. Despite his hesitance, Hotsuma did as he was instructed and killed his brother to end the ritual. Little did he know, this action initiated a series of events that would destroy his clan and bring an immense sorrow upon the world.
Soon after his awakening, Hiruko laid siege to the Oboro village, both as an act of revenge and to initiate his plan to acquire ultimate power. Unfortunately, the sword was nowhere to be found in the village, so he sent his Hellspawn to search for it. Ultimately, he was informed that Hotsuma had it in his possession. He used his dark magic and gojufu to summon Hellspawn and reanimate the corpses of the slain Oboro and sent his legion of soldiers to wreak havoc on the world. His ultimate plan, to kill Hotsuma and retrieve the sword, was underway.
In the end, Hotsuma rose to Hiruko's challenge and sought out his revenge. He cut down legions of Hellspawn and the corpses of his fallen comrades. He destroyed the Hellspawn weapon, Yatsurao, and defeated Hiruko atop his Golden Palace.
- Haku is a component of Yin that remains in the body. When a being is killed by Akujiki, the blade sucks up their corporeal essence and their physical bodies vanish.
- When the player performs a tate (殺陣), the fallen enemies will release three times more Yin than when they are killed with a normal attack.