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I have followed Naruto only via its manga and, barring a few episodes, I haven't seen much of the animated series. Considering that the manga ended almost a year ago, I am very much out of the Naruto universe.
Few months ago, I came across some posts that mentioned Itachi getting a separate anime series of his own, with trailers of the series uploaded on YouTube. Today I saw some amvs titled Itachi Shinden and a part of the episode which has Shisui and kid Itachi fighting the ANBU.
A few internet searches yielded that Itachi Shinden is actually an animated adaption of the novel of the same name. However, I couldn't find whether it's a separate series that was being talked about or part of Naruto Shippuden fillers.