While Itachi can use Tsukuyomi with his left Mangekyo Sharingan, he can use Amaterasu with his right. The black flames of Amaterasu burn the target until it is completely incinerated. Water and other methods of extinguishing fire do not work on the black flames.
Ways to evade Amaterasu include being fast enough to dodge it, absorbing it, and blocking it with chakra. However, many characters in Naruto do not have the means necessary to avoid it, so Amaterasu is an extremely useful weapon at Itachi's disposal The skill was adapted from the PS2 game
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