The first person to discover the power of the Kagirinaigan was Rai Kuraihi. He then passed the power onto his grandson, Hikaru. He also found out an interesting fact about the Kagirinaigan while in a shrine that had information on this new power. Rai read into the inscription, reading the ancient text:
The limit for the Kagirinaigan is 3 Soul Reapers per century.
It also read further in the inscription that a certain clan can obtain the Kagirinaigan. It said something like this:
The legendary power is a blessing for the clan who can use a blade of shadows. If there was to be one who took the legendary power, or if the person wielding the legendary power, and was not of the clan who can use a blade of shadows, that person shall immediatly die. If the thief, or person is not of the clan who can use a blade of shadows, is an immortal, they shall be stripped of their power, and cursed to walk the earth for 1000 years. They shall also be given a cursemark, so that no matter who they meet, all shall shun them, until the cursed person repents of their sin to The One Who Is Holy. Only then may they be returned to their previous status.
It also was carved at the lower area of the inscription: Seireitou, this means you!
One of the most notable abilities is being able to track all movements. A fully mature Kagirinaigan can track all movements, no matter how fast. But to date, no one has developed a fully mature Kagirinaigan.