Daggers of Westernesse
The Daggers of Westernesse were forged by the Dúnedain of Cardolan during their war against the Witch-King of Angmar. They were apparently made for fighting the barrow wights sent from Angmar, and they had the special property to harm wraith-like spirits such as the barrow wights and the Nazgûl. After the fall of Cardolan, at least four of these daggers ended up in the tomb of the last prince of that realm.
In the Fellowship of the Ring, Frodo, Sam, Merry, and Pippin became lost in the Barrow Downs and were captured by one of the wights and imprisoned in the tomb. Frodo was able to call Tom Bombadil to their aid, and he opened the tomb and drove the wight away. Tom advised the hobbits each to take something with them from the tomb, and gave each of them a dagger. These weapons proved useful later in their journey.
One of these daggers played a special role in The Return of the King. During the Battle of the Pelennor Fields, Meriadoc Brandybuck stabbed the Witch King in the back of the knee with his dagger. Many fans speculate that the spells on this dagger made the Witch King vulnerable to Éowyn's more mundane sword, which allowed her to kill him.