Tsukuyomi is a genjutsu that places whoever Itachi establishes eye contact with in an illusion where Itachi can control the passage of time freely. The victim can spend years within Tsukuyomi, and only a few moments would pass in real time. He usually uses this time to torture victims and break their minds, kill them in certain cases, place mental trauma on them, show them any illusions of his choosing, or simply mind control them.
The target's body is completely paralyzed while under its effects, but it's so fast in real time that it doesn't really matter.
Itachi also has an assortment of other genjutsu at his disposal, but Tsukuyomi is by far his most powerful.
As for the Totsuka Blade, it's an intangible blade that seals away whatever it pierces into a genjutsu world contained in a Sake gourd.
Itachi obviously also has all the regular Sharingan shenanigans, such as prediction, power mimicry, info analysis, enhanced sight, illusions and mind control, etc.
He can use clones to scout the enemies' abilities, gain information, or cause distractions. He can also make them explode.
He also has Amaterasu, which ignites whatever he's looking at with flames that can't be extinguished and ignore durability. The only way to avoid it is to escape the focal point of his gaze before the Jutsu is cast.
He's very intelligent, cautious, and pragmatic, and usually assesses the situation accurately to exploit any weaknesses his enemy may have.
Not gonna vote, but thought I'd explain some of Itachi's abilities for the memes. You're welcome.
Also everything I just said GG!!!!!!!!!!