Morgul-knife

Morgul blades are magical, poisoned daggers used by the Nazgûl.

At Weathertop, during his journey to Rivendell with the One Ring, the Hobbit Frodo Baggins was stabbed by the Lord of the Nazgûl. A fragment of the blade remained within the wound, working its way toward his heart and threatening to turn Frodo into a wraith. Elrond was able to remove the shard and heal the wound, but each year on the anniversary of his stabbing Frodo became seriously ill. Only his eventual departure to Eldamar offered a permanent cure.

Athelas (or Kingsfoil) is known to slow the poisonous effect of the morgul-blade. This treatment is also effective against other Mordor illnesses, such as the Black Breath.

Another victim of a morgul blade was Boromir, the eleventh Steward of Gondor (not the Boromir of Frodo's company). He eventually died of his wound but did not become a wraith.

Origins

In the Middle-earth Role Playing games, the Morgul Blades are said to have been forged in the fires of Minas Morgul by the Witch-king of Angmar and he embalmed them with his dark sorcery, but this is considered non-canonical as it does not appear outside of the role playing material.