The Last Alliance of Elves and Men was a pact formed by Elendil, High King of Gondor and Arnor, and Gil-Galad, High King of the Elves in Middle-earth, in response to the growing power of Sauron, now returned from the ruin of Númenor. Initially composed of the forces of Lindon and Arnor, the main army stayed in Rivendell for three years in order to strategise, train, and forge the weapons needed to fight the forces of Mordor. Then they climbed over the Misty Mountains and moved south along the River Anduin, where they were joined by forces from Moria, Lothlórien, Greenwood (later Mirkwood), and finally Gondor.
The Army then marched to Dagorlad, where they defeated the forces of Sauron in a titanic battle and continued on for a grueling ten-year siege of Barad-dûr, in which Elendil's younger son, Anarion, was killed by a thrown rock. In the end, Sauron himself came out of his tower to duel Gil-Galad and Elendil in a fierce battle that ended in the death of the two of them. Isildur, the remaining son of Elendil, then cut off Sauron's fingers, causing him to lose possession of the One Ring, and therefore his power. This ensured the victory of the Last Alliance, though at a staggering cost.
Isildur later decided to keep the Ring, thus ensuring Sauron's survival and deciding the course of the next few millenia.
Fanfics dealing with the Second Age of Middle-earth are rare, and fics dealing with its end are rarer. Thus, there is scant material on the Last Alliance, despite its climactic status in the history of Middle-earth.