Yin (in Japanese: 魄 Haku; Yin Energy) is a dark energy that inhabits every soul from sin and negative emotions. Akujiki consumes this substance from every creature and person the blade kills. Hiruko Ubusuna needed the Yin to gain power.
Performing a Tate will give three times as much Yin compared to a normal attack.
The Yin serves the hunger of Akujiki, the evil soul-eating sword. The more it kills, the more yin it consumes.
Hotsuma, now the leader of the Oboro Clan, is forced to abate Akujiki's appetite for the yin. He uses the blade to destroy the Hellspawn and soon Hiruko who uses the yin of his victims and his puppets to gain more power.
The Yin is the opposite of Yang.
- In contrast to the English translation used in Shinobi, Haku is the Yin side of the spirit that remains in the body. hence why when someone dies from Akujiki's blade, the blade sucks up their corporeal essence and their physical bodies vanish.