Yuei was the first to develop the jutsu that would allow him to locate spies that infiltrated or left the village. He knew that he could not allow his clan's biggest secret, his father, be exposed to the world, or they would be hunted and slaughtered mercilessly. Knowing that he would need to mutate the Hikari's blood to be able to make this useful for following assassination targets, he began to work on a tonic that would be permanently bonded to the blood. In more recent times, the Hikari use this to track animals back to their dens so they can gather more meat from their hunts, or perhaps be led to a hidden orchard.
You must get your blood inside of your opponent, and then preform the required hand-seals.
If a drop of your blood gets into the opponent's bloodstream, your blood can feel them, provided that it does not get mistaken for a virus and is destroyed by the white blood cells in their body. Mostly used for tracking enemies or animals, The Hikari tend to use this skill quite often, especially when chasing enemies who seem to slip out of their hands.
- The ability to track any shinobi, no matter how far they are or how many worlds apart they are from you.
- Can be extremely subtle; Example; serving as a waiter/waitress in a shop, you can easily prick your finger and put a drop of blood in their food and or drink.
- Must be inside of the Target's bloodstream. Can sink through skin on contact, but extremely difficult to hit a moving enemy with.
- The taste of iron can be detected, as the blood of the Hikari has a stronger taste and smell than normal blood.
- Handseals have to be executed after blood has entered the target's bloodstream.
- The more Targets are infected, the harder it becomes to differentiate between them since the technique offers no manner of saying "who's who.".