After returning to Nathanos in Dazar'alor, he ensures the information is sent to Sylvanas, and sends us to our next mission: SI:7 activity has been detected in Stormsong Valley, so we're sent there in Securing Warfang Hold. Arriving there, the leader of the settlement, Boss Mida, mentions that Nathanos is correct, but don't tell him about the situation or he will get all smug about it. You then help control the Alliance spies by slaying them in Spy Games and destroying their listening devices in Communication Breakdown.