The Water of Life is a liquid medicine developed by Doctor Yukimura Kodo to turn humans, and demons, into furies. It was created with ingredients from the West, including the blood of Western demons, and diluted with water from the Yukimura village in Mutsu, Shirakawa.
After Kodo joins the Imperialist forces, Sanan takes on the task of resuming his abandoned research and refining the Water of Life. By doing so, he reduces its negative effects and makes it usable within the Shinsengumi, in accordance with the shogunate's orders.
In the games Edit
The Water of Life is first hinted at in Chapter 1, shortly after Inoue informs the rest of the Shinsengumi that Sanan has been injured. Okita states that "if push comes to shove, he'll have to take it", but does not explain any further.