Ok so first of all many people are saying that its probably Genjutsu but i doubt thats likely because we all know by now that sharingan can dispel genjutsu and we can see that Danzo is no novice in using his sharingan effectively.
So some people say that Itachi is a part of the Tsukiyomi but so far every single time someone used Tsukiyomi the target was warped into some sort of alternate dimension with distorted time and space, and clearly the surroundings still seem to be the same.
Danzo seems suprised at the sight of Itachi and so far in this fight Danzo was not suprised or even slightly faltering even when he was in the grasp of Sasuke's Susanoo so we can tell that Danzo is genuinely mystified at Itachi's sudden arrival so i doubt that Itachi is a product of Danzo's tricks or even Madara's becuase the Crow's came out of Sasuke not Danzo.
So given all this i have a few ideas on what Itachi's appearance could be.
1.) Itachi could be a triggered thing like Minato in Naruto that was set to activate when Sasuke started fighting Danzo( which showed that he was trying to destroy Konoha) and mightve been a safeguard in case Sasuke chose that evil path to destroy Konoha and its Elders.
2.) another thing that no one was mentioned is that this appearance of Itachi could be a new Mangekyou ability of Sasuke. It was a previous theory that the number of parts the sharinghan divided the eye into the number of mangekyou abilities the user would possess and so far we haven't seen any of Sasuke's original mangekyou abilities. And based on the story every person to have possessed the Mangekyou has shown at least one different ability. Itachi: Tsukiyomi, Amaterasu, and Susanno. Madara with his space/time jutsu and ability that allowed him to cheat Death from the 1st Hokage. So its only up to reason that even if sasuke doesnt get 6 total mangekyou abilities he should get atleast one original Mangekyou ability of his own.
So its possible that this might be his special ability as we see Sasuke evidently concentrating with Magekyou eyes open. Now due to the strain of the mangekyou, Sasuke would have to be a dumba*s to just waltz around with both mangekyou eyes open and not be doing a jutsu so this might be the product of that double eye stare
1.)Thou that might be true every tried considering the fact that sasuke was the former pupil of orochimaru and during the fight of orochimaru and the 3rd Hokage, orochimaru used a forbbiden ninjutsu which brings back the dead in order to fight for the summoners. Sasuke probably could have learned this technique from his years studying under orochimaru.
2.) Look closely at the page before u see itachi. u see sasuke diverging into crows. Itachi might have given something more than just amaterasu, tsukiyomi, susanoo, and MS. Its possible itachi might have sealed some of his life force inside of sasuke just like the forth hokage.
3.) Before itachi went to fight he gave naruto some of his powers and he quotes " I hope the day never comes when you will have to use this" he might have meant he would not want to face sasuke after decieving him while he tries to take revenge on the 3 remaining elders who know of the truth. Itachi might have learned/created a jutsu which needs 2 factors
sasuke::naruto
to implant life force with in, in order to work.
Conclusion: Consider these facts respond to me and then we will figure the delima out!
