An ancient dagger was an ancient island artifact used in the attempt to kill the Mother, Jacob, and the Man in Black, but proved successful only in the killing of the Mother. This dagger may have had special properties, since two characters specifically presented it as the weapon that must be used for the killing of these three, as well as other stipulations.
According to the Man in Black, and 140 years later Dogen, whoever wielded the dagger must plunge it into the chest of the victim (Jacob and the Man in Black respectively) before any words were exchanged in order to successfully kill them. It is unknown if this stipulation was literal or figurative. ("Sundown") ("Ab Aeterno") Due to its failure to kill the Man in Black, as well as the fact that Jacob was later killed with a different knife, it is highly unlikely that the dagger was anything more than a normal weapon that the Man in Black had owned since around 43 AD. This explanation also makes it likely that the stipulation that the target must not be allowed to speak was due simply to the charisma both Jacob and the Man in Black possessed, and thus the possiblity that they could convince their assailant not to stab them.
The dagger's origin is unknown; however, it resembled a pugio (Roman military dagger), which suggests that it arrived as part of the manifest of Claudia's ship when it crashed on the Island. ("Across the Sea") The Man in Black used it to demonstrate to Jacob the strange electromagnetic properties that his people had discovered in various places on the Island, by throwing it and thus demonstrating how it was attracted magnetically to the stones of a nearby well. Later, the Man in Black used the dagger to stab Mother in the back and kill her; she did not anticipate this assault and thus did not speak prior to it. ("Across the Sea")
The dagger came to be in the possession of the Man in Black. Upon convincing then devout Catholic Richard Alpert that they were in hell and that Jacob was the Devil, the Man in Black informed him that he would have to use the dagger to kill Jacob if he ever wanted to see his wife Isabella again.
The Man in Black instructed Richard to kill Jacob in the same way that Dogen would later instruct Sayid to kill the Man in Black: by plunging the knife deep into his chest, before he could speak a single word. After telling Richard that "if Jacob speaks, it will already be too late," the Man in Black added, "He can be very persuasive," perhaps providing some evidence for the theory that this stipulation was figurative only. When Richard arrived at the ruined statue of Taweret, he was disarmed in a fight with Jacob, who seemed to remember the dagger that had killed his mother, asking Richard, "Who gave you this?" (It is likely that he already knew the answer.)
After Richard explained himself, Jacob convinced him that he was not the devil, that Richard was not in hell, and that the Man in Black was an evil entity. ("Ab Aeterno")
Jacob apparently never returned the knife to the Man in Black; it was next seen in the possession of Dogen.
After Claire entered the Temple, Dogen summoned Sayid and told him to stab the Man in Black in the chest with the knife, before the Man had a chance to speak. It was stated that once the Man in Black spoke, it would be too late. After the Man in Black greeted Sayid, Sayid directly stabbed his chest, which had no effect - however, it is unknown whether it was because he spoke first, because just a day earlier bullets were also seen having no effect on the Man in Black. He claimed it was a ruse by Dogen to have him kill Sayid.
When confronted by the fact that the knife didn't work, Dogen did not deny the ruse. He was drowned by Sayid, who then used the knife to slit Lennon's throat when the translator discovered the aforementioned murder. ("Sundown")
- The knife was a pugio, a military dagger used as a sidearm by Roman legionaries. On the sheath, it depicted the Roman she-wolf suckling Remus and Romulus.
- Knives - an overview of other knives on the show
- Locke's knives - Locke's set of knives
- Jacob's knife - a different knife used by Jacob