Kakashi's Anbu Arc: The Shinobi That Lives in the Darkness

The is the seventh filler arc from Part II of the series and spans from episode 349 onwards. The arc focuses on Kakashi Hatake's ANBU background, taking place after the battle at the Kannabi Bridge, losing his closest friend Obito Uchiha, and later taking Rin Nohara's life with his own hands.