Akumu Uchiha

Around the end of Uchiha Dynasty, Nightmare used the Gem of Helios to copy the chakra from Itachi to gain the Sharingan in both of the Soul Edge and Nightmare, which transformed Nightmare into the demonic clone of Itachi that is called Akumu Uchiha.

Trivia:
 * Through he is not a real member of the Uchiha clan, he called himself one since he was the clone of Itachi.
 * Akumu is japanese for Nightmare.