"Shisui was one of the few members in the clan's history that was able to awaken the Mangekyō Sharingan. It granted him access to powerful genjutsu techniques, the strongest of which was a unique genjutsu noted to be of the highest calibre: Kotoamatsukami. This technique allowed him to enter a person's mind and manipulate them by giving them false experiences, making it seem as if they were doing things on their own free will; the victim wouldn't even notice they were being manipulated. This ability was apparently so unique and powerful, that Tobi wanted it for himself and was angry that Danzō managed to destroy it before he died. Kabuto was also extremely thrilled when he learned that the remaining eye was within Itachi's crow. This genjutsu however, could only be used once a decade, unless the user had a way to augment the process such as with Hashirama Senju's cells, thereby drastically reducing the time required from ten years to only a few hours. Shisui's eyes were also seen as something of a treasure even within his own clan, as before his death he had to ensure that it looked like he had destroyed them before he had died, fearing it would end in conflict otherwise."
-Quoted from Narutopedia