Remember the Uchiha Clan Massacre?
Of course you do. It happened because the Uchiha were planning a Coup to take over the village. Itachi, who was a double agent, had already informed the Leaf of the Coup.
Shisui, meanwhile, was planning to use Kotoamatsukami on Fugaku Uchiha, and stop the Coup by himself. But, Danzo interrupted, and took away one of his eye. Danzo would later use this eye for his own good.
Meanwhile, Shisui gifted the other eye to Itachi and killed himself. The entire burden fell on Itachi. But, if Shisui could’ve used Kotoamatsukami on Fugaku, would he have succeeded? I’ll answer this question in this post.