An accursed blade carried by the Nazgûl. When a victim is stabbed with a morgul-knife, the point breaks off leaving pieces of the blade in the wound. If the injury itself does not kill, the spell on the blade slowly works its evil on the victim.

In the year 3018 of the Third Age, Frodo was stabbed with such a blade atop Weathertop and almost became a Wraith. Luckily for him, the last shard of the blade was found and removed and he was healed by the skills of Elrond. Even so, the injury never completely healed and bothered Frodo for the rest of his days on Middle-earth.

In the wars against the Witch-king, Boromir I, one of the Stewards of Gondor, was also pierced by a morgul blade. This wound shortened his days and he ruled Gondor for a mere 12 years after the death of his father, Denethor I. He died a shrunken man, racked with pain in the year TA 2489.
Encyclopedia entry originally written by PotbellyHairyfoot