Narsil was the sword of Elendil, King of Arnor, father of Isildur, who was ruler of Gondor. It was forged in the First Age by Telchar. It was broken in the War of the Last Alliance, as Elendil engaged in personal combat with the Dark Lord Sauron. Isildur used the broken shard of Elendil's sword to cut the One Ring of Power from Sauron's hand.
The sword was reforged under Aragorn for use in the War of the Ring, at the close of the Third Age. At this time, the blade was renamed Anduril.
The name narsil comes from the word stems -nar and -thil, meaning fire and white light. As a result, there is some speculation that narsil blazed up in battle:
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 (from the Silmarillion).