How did Shisui awakened his Mangekyo Sharingan?


We know that to awaken Mangekyou Sharingan the wielder must witness the death
of someone close to them like Itachi with Shisui's death, Obito and Kakashi
with Rin's, and Sasuke with Itachi's.

So whose death did Shisui see?




Per the [wiki](http://www.naruto.wikia.com/wiki/Shisui_Uchiha): "At the end of
the Third Shinobi World War Shisui witnessed his best friend die without being
able to do anything to save him, consequently awakening the Mangekyo
Sharingan" The reference is "Itachi Shinden: Book of Dark Night" (which I
skipped).




Per the [wiki](http://www.naruto.wikia.com/wiki/Shisui_Uchiha): "At the end of
the Third Shinobi World War Shisui witnessed his best friend die without being
able to do anything to save him, consequently awakening the Mangekyo
Sharingan" The reference is "Itachi Shinden: Book of Dark Night" (which I
skipped).

