I'll expand on this more later but I think Itachi used tsukiyomi on himself in order to acquire knowledge on ancient artifacts relating to totsuka and yata.
In the tsukiyomi world, 3 real world seconds=3 days. Itachi could have abused this in order to acquire vast amounts of knowledge and even refine his skills as a ninja. Honestly, it would explain why a 21 year old was in possession of 2 ancient spiritual weapons that Orochimaru, a man three times his age, whose intelligence and dedication to studying the secrets of the ninja world were almost unrivaled, couldn't find them.
It could also be a huge reason why Itachi went blind despite only using his mangekyou 3 times in the manga prior to fighting Sasuke in part 2. The abuse of tsukiyomi could also be the reason for his mystery illness.
In the tsukiyomi world, 3 real world seconds=3 days. Itachi could have abused this in order to acquire vast amounts of knowledge and even refine his skills as a ninja. Honestly, it would explain why a 21 year old was in possession of 2 ancient spiritual weapons that Orochimaru, a man three times his age, whose intelligence and dedication to studying the secrets of the ninja world were almost unrivaled, couldn't find them.
It could also be a huge reason why Itachi went blind despite only using his mangekyou 3 times in the manga prior to fighting Sasuke in part 2. The abuse of tsukiyomi could also be the reason for his mystery illness.
