To verify, Kiritsugu took out an eye drop bottle from his pocket containing the liquid that could be used to test his hypothesis. The liquid was made by refining the body fluids of a succubus, a kind of demon that particularly liked to seduce men. It was especially sensitive to the blood of men and aged things, and could distinguish differences accurately.

First he tested the reaction of the liquid in the bathroom sink, and then he verified the bloodstains in the living room. It was evident that the reaction was the same for both. Only one man would have shaved at this bathroom sink in the past few days.

And that man’s blood was spilled on the carpet in the living room…

Now he was sure Tōsaka Tokiomi was either dead or had disappeared.

- Fate/Zero: Act 14, Part 4