According to this Naruto theory, Itachi's susanoo ability, the Totsuka Blade is an advanced and modified form of the Reaper ...

Naruto has some of the coldest aura-farming moments in the history of anime, and the biggest aura-farmer isn't Itachi Uchiha.

Likewise, Itachi reveals his trump card, Uchiha’s hidden techniques, Amaterasu and Susanoo. Unfortunately, his disease-ridden body couldn’t withstand the weight of such a powerful technique.