i believe sasukes last move was a form of chidori, turned into a strike of lightning. he made a storm with a fire tech. then used the electricity to power up his chidori. after that itachi was still standing. itachi walked up to sasuke slowly then simply poked his forehead like in the past. then itachi collapsed. and died ( >.< ) due to the disease caused by the mangekyo sharingan.
The simplest way of putting it is...
Sasuke absorbed Orochimaru, who which a part of escaped into Kabuto. Then during the fight between Sasuke and Itachi, the part of Orochimaru inside Sasuke was brought out and killed. Then Itachi died fighting Sasuke.
After that, the ninja world went to hell.