Great Signal

The Great Signal, as Gorbag the Orc called it, occurred on 10 March T.A. 3019. On that date, as Sam and Gollum urged the weary Frodo to stand up and start climbing the Stairs of Cirith Ungol, the ground shook and a vivid, violent red flash leapt up from within Mordor, painting the clouds crimson, followed by a crack of thunder. The Signal was answered by Minas Morgul; blue lightning forked from the tallest tower and surrounding hills into the dark clouds.
The Great Signal was the call for the armies of Mordor to begin the assault upon Minas Tirith. As the hobbits and Gollum watched the gates of Minas Morgul opened and an army came forth, led by the Lord of the Nazgûl. At the same time another army issued from the Morannon heading for Cair Andros, probably set in motion by the same Great Signal.