Narsil was the sword of Elendil, High King of Arnor and of Gondor. It was forged already in the First Age by the renowned dwarven smith Telchar of Nogrod. Handed through a line of Noldor it was probably given to Elros, an half-Elf and early ancestor of Elendil.
About Narsil (Quenya for Red-white Flame) Tolkien wrote that the name referred to the "chief heavenly lights, as enemies of darkness" which would be the sun and the moon.
Elendil brought Narsil with him when he escaped the Downfall of Númenor and he used the sword throughout his life, until it broke under him at his death Second Age 3441, at the Siege of Barad-dûr. At that point, when Elendil and Gil-galad had overthrown Sauron but both died, Isildur took the hilt of Narsil and cut off the One Ring of Sauron's hand. The shards were collected and kept safe as an heirloom of Dúnedain.
In Third Age 2, once Isildur had restored order in Gondor he journeyed towards Arnor with the shards of Narsil and other valuable items. To bring with him his wife and youngest son he took the route via Imladris (Rivendell) and was suddenly attacked by a large host of orcs. His three sons and all of his valiant men, save three, perished in what is known as the Battle of the Gladden Fields.
Realizing the grim destiny Isildur commanded his esquire Ohtar, and his companion, to at all costs bring the shards to safety. They managed to do so and reached Imladris where the shards are stored yet today.
These shards are reforged in an epic episode and given to Aragorn. The reforged sword is renamed to Andúril, the "Flame of the West", and will surely be wielded by Aragorn when he eventually attacks the Enemy in a future, unavoidable battle.
- The Lord of the Rings, by Tolkien
- The Letters of J.R.R. Tolkien