|Location||Mordor, Minas Morgul|
|Date||10 March T.A. 3019|
|Result||The Hosts of Sauron march|
|Part of||War of the Ring|
|Description||A violent red flash from MordorBlue lightning from Minas Morgul|
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.
 Portrayal in adaptations
- In Scene 16, "Minas Morgul", the Great Signal occurs just after Sam and Gollum have prevented Frodo from walking straight up the road to the gates of the city. There is no eruption of light from Mordor; the signal consists of a swirling twister of luminous vapor shooting into the clouds above the central tower. The scene cuts to Minas Tirith where Gandalf and Pippin witness the Signal. Frodo, Sam, and Gollum rush back to the base of the stairs and hide behind an outcropping of rock. Soldiers and citizens of Minas Tirith are shown silently reacting in alarm to the sight. The Lord of the Nazgûl appears on a Fell Beast and the Orcs march from the city.
- In J.R.R. Tolkien's text, the Great Signal happens three days after the lighting of the Warning beacons of Gondor. In the movie the sequence is reversed; in the morning after the Great Signal Gandalf sends Pippin climbing up to an unlit beacon looming over the City where he ignites it.