Battle of the Morannon
This was the final major battle against Sauron in the War of the Ring, fought at the Black Gate of Mordor. The army of the West, 7,000 strong by now, led by Aragorn marched on the gate as a diversionary feint to distract Sauron's attention from Frodo and Sam, who were carrying the One Ring through Mordor. It was hoped that Sauron would think Aragorn had the Ring and was now trying to use it to overthrow Mordor.
Initially, the Army of the West had been composed of 8,000 men, but as they approached Dagorlad near the Black Gate Aragorn dismissed the faint of heart, ordering them to liberate Cair Andros on the river Anduin. This resulted in the departure of maybe 500 men.
Before the battle began, Sauron sent one of his servants, the Black Númenórean called the Mouth of Sauron, to speak with the Captains of the West. He tried to trick Gandalf into believing Sauron held Frodo captive, displaying as evidence items that had belonged to Frodo and Sam (Sam's sword, an Elven cloak, and Frodo's Mithril shirt.) The Mouth threatened that Frodo would be tortured if the West did not agree to Sauron's terms of surrender. (It is clear that while Sauron knew there was a Hobbit in Mordor, he did not know why.) Gandalf, however, refused to be swayed, took the items from the Mouth of Sauron, and sent him away. Amazed and angered, the Mouth of Sauron rode back to the Black Gate, let it be opened, and the forces of Sauron marched out. At the same time, more of Sauron's forces that had been hidden in the hills around the Black Gate came forth, thus surrounding the Men of the West. Sauron's army outnumbered that of the West by at least ten to one.
Against Aragorn's army was arrayed Sauron's hordes of Orcs, Trolls, and barbarian Mannish allies such as the Easterlings and Southrons (Haradrim). An exact count is not given of the number of Sauron's forces, and though they numbered in the tens of thousands at least, the battle is said to not have been as quite large as the preceding Battle of the Pelennor Fields.
During the course of the battle, the eight Nazgûl still alive attacked the army of the West. The Eagles of the Misty Mountains, led by Gwaihir the Windlord, arrived and attacked the Ringwraiths. At that moment, when all hope seemed lost, Frodo put on the One Ring and Sauron realized that Frodo was inside Mount Doom. The Nazgûl immediately left the battle to intercept Frodo. However, Gollum bit the Ring off Frodo's finger and then accidentally fell into the Crack of Doom, and Sauron's power was overthrown.
The Nazgûl had been flying over Mount Doom just as it underwent a gigantic volcanic eruption, and they were all destroyed in the firestorm. Barad-dûr, the Black Gate and, the Towers of Teeth collapsed to ruin. The Orcs and other creatures of Sauron were completely directionless with the Dark Lord's demise and were easily decimated by the army of the West. The Easterlings fought on stalwartly, though eventually many threw down their weapons and surrendered (later to be sent home in peace by Aragorn King Elessar).
Fighting against Sauron's remaining forces would continue in the northern theater of the War of the Ring for several weeks, notably at Dol Guldur in Mirkwood and at Erebor, but the power of the Dark Lord of Mordor was no more.