short summary: Atasuki has almost been completely destroyed. Sasuske 'killed' (more like absorbed) Orochimaru, then killed his brother, while Naruto and the others were also killing some of them. The frog guy died fighting their leader.
After killing his brother, another dude from Atasuki informed him that Itachi didn't murder his family out of spite, but it was a mission from Kohana because the family was planning a rebellion. He spared his brother, hoping he'd track him down and kill him some day.
Sasuke gets all emo over it, joins up with Atasuki to help destroy the leaf village. To do that they need the black ninja's demon, as well as Naruto's.