As usual, the Disguise will not work if you have any run-speed buffs on. Also, you must "change" inside the tent that is between the Second Aegis and the cave where the meeting will take place.
The meeting is between Commander Calthyx of the Abyssal and Ambassador Thaelia of the Storm Legion. The Storm Legion is out to recover the Frozen Soldiers, and the Abyssal are helping by supplying thawing devices. In exchange the Abyssal will keep all the Tidestone they find.