The Last Alliance of Elves and Men was formed by Gil-galad, the Elven King and by Elendil, the King of Gondor, the last leader of the Númenóreans in Middle Earth.
"...Elendil and Gil-galad took counsel together, for they perceived that Sauron would grow too strong and would overcome all his enemies one by one, if they did not unite against him. Therefore they made that League which is called the Last Alliance, and they marched east into Middle-Earth gathering a great host of Elves and Men; and they halted for a while at Imladris. It is said that the host that was there assembled was fairer and more splendid in arms than any that has since been seen in Middle-earth, and none greater has been mustered since the host of the Valar went against Thangorodim.
From Imladris they crossed the Misty Mountains by many passes and marched down the River Anduin, and so came at last upon the host of Sauron on Dagorlad, the Battle Plain, which lies before the gate of the Black Land. All living things were divided in that day, and some of every kind, even of beasts and birds, were found in either host, save the Elves only. They alone were undivided and followed Gil-galad. Of the Dwarves few fought upon either side; but the kindred of Durin of Moria fought against Sauron.
The host of Gil-galad and Elendil had the victory, for the might of the elves was still great in those days, and the Númenóreans were strong and tall, and terrible in their wrath. Against Aeglos the spear of Gil-galad none could stand; and the sword of Elendil filled Orcs and Men with fear, for it shone with the light of the sun and of the moon, and it was named Narsil...
There in the valley of Gorgoroth Anárion son of Elendil was slain, and many others. But at the last the siege was so strait that Sauron himself came forth; and he wrestled with Gil-galad and Elendil, and they both were slain, and the sword of Elendil broke under him as he fell. But Sauron was also thrown down, and with the hilt-shard of Narsil Isildur cut the Ruling Ring from the hand of Sauron and took it for his own. Then Sauron was for that time vanquished, and he forsook his body, and his spirit fled far away and hid in waste places; and he took no visble shape again for many long years..."
-J.R.R. Tolkien, The Silmarillion p. 293-294.
Elrond recounts to Frodo his part in the Last Alliance.
"I was the herald of Gil-galad and marched with his host. I was at the Battle of Dagorlad before the Black Gate of Mordor, where we had the mastery... I beheld the last combat on the slopes of Orodruin, where Gil-galad died, and Elendil fell...
On the taking of the One Ring by Isildur:
"Isildur took it, as should not have been. It should have been cast then into Orodruin's fire nigh at hand where it was made. But few marked what Isildur did. He alone stood by his father in that last mortal contest; and by Gil-galad only Círdan stood, and I. But Isildur would not listen to our counsel...soon he was betrayed by it to his death; and so it was named in the North Isildur's Bane...Narsil was broken and its light extinguished, and it has not yet been forged again."
-J.R.R. Tolkien LOTR:FOTR p. 237.
