The Meeting of the Masters is about to commence, and you have arranged for a disguise that should allow you to attend in secret.
'You have returned just in time, for it seems the meeting is soon to commence. Spies have reported large enemy groups moving toward Orodruin from different strongholds in Gorgoroth. They appear to be making for Amon Thaur, a structure on the side of Mount Doom.
'Are you prepared? Speak to me when you are ready to leave, but do not take too long! Time is short, and you must not miss this meeting!'
Mithrandir is at Cirith Gorgor, just inside the Black Gate.