The Barrow-blades were daggers from the Barrow-downs, given by Tom Bombadil to the four hobbits of the Fellowship. And in the movie (the fellowship of the ring) the blades were given by strider (Aragorn)
The Barrow-blades were found in the barrow of a Barrow-wight on the Barrow-downs and given to the hobbits by Tom Bombadil, who told them that the blades were forged long ago by Men of Westernesse, foes of the Dark Lord who were overcome by the evil king of Carn Dûm in the Land of Angmar.
The Barrow-blades were described as "long, leaf-shaped, and keen, of marvelous workmanship, damasked with serpent-forms in red and gold. They gleamed, ... wrought of some strange metal, light and strong, and to set with many fiery stones." Whether because of protection from their black sheaths or because of the spell that lay on the Barrow mound or the alloy used to make them, "the blades seemed untouched by time, unrusted, sharp, glittering in the sun."
Frodo's sword broke in the confrontation with the Nazgûl on the banks of the River near Rivendell and was replaced at Rivendell by Bilbo's gift of the elvish knife Sting. Sam, Merry, and Pippin kept their swords. The Barrow-blades carried by Merry and Pippin were taken and discarded when the two hobbits were captured by Uruk-hai at Amon Hen, but Aragorn saved them and they were returned when they were reunited at Isengard. Merry's Barrow-blade played a major role in the Battle of the Pelennor Fields. It was enchanted with the power to harm the Witch-king of Angmar himself by a weaponsmith of Arthedain long before. When he stabbed the Witch-king in the knee with it, it distracted the Nazgûl and broke the spell binding his undead flesh to his will, allowing Eowyn to kill him by driving her sword into his unseen head, thus fulfilling the prophesy of Glorfindel that "not by the hand of man shall he fall." The sword is approximately 30 inches long, so as to be weilded by Hobbit-folk.
Portrayal in adaptationsEdit
In the film adaption, Aragorn not Tom Bombadil gives the hobbits the barrow-blades. Also, Merry and Pippin never get their barrow-blades back from the orcs, and it's presumed that their swords are still in the orc skeleton pile. Merry gets a new sword from Theoden. Pippin gets a new sword from Denethor which could have belonged to Faramir or Boromir. Both are presumed to be correct as Pippin's clothes belonged to Faramir, but Pippin served Denethor as a replacement for Boromir. To pressure Pippin's debt he could have given him Boromir's sword. Also the sword could be a simple barrow-blade. Also in the movie, Frodo's barrow-blade is not destroyed, but when the Nazgûl confront him, he simply drops it. Bilbo just gives Sting as a gift and it's unknown what happens to Frodo's. As Frodo or anyone else didn't take it with him when they started the Quest of the Ring, it either stayed in Rivendell or was left on Weathertop. The latter is more presumed as after Weathertop, Frodo's blade is never seen again.