The Morgul blade is a magical poisoned dagger.
At Weathertop, during his journey to Rivendell with the One Ring, the Hobbit Frodo Baggins was stabbed by the leader 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.
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 Canon as it does not appear outside of the role playing material.