Originally Posted by kael03
We're assuming it was a sealing dagger (I'm playing devil's advocate for this one part). That tag could've been an explosive tag as well. But, if it was a sealing tag, I'm betting it was the earlier tag Kabuto pulled out to rewrite Itachi's brain...again in order for Kabuto to regain control.
Kabuto stated twice he means to regain control of Itachi....the kunai aka tag is to do just that...