The Ancient Fang is a magical dagger from Innistrad that holds memories from everyone that has used it. This dagger is very powerful - it can cut wooden walls as though they were made from butter, it can ignite things, or kill the undead.
The Ancient Fang was once used by the planeswalker Sifa Grent while murdering the people of Drakeston on Fiora. Sifa gave the dagger to the Cult of Rakdos on Ravnica for several reasons, chiefly to secure their loyalty, but more to keep it away from those who might want it on the plane from which it had originated.
The dagger was consequently stolen by the thieving planeswalker Dack Fayden, who himself hailed from Drakeston. By the magic of the dagger he accidently found out who was responsible for the death of everyone he had known. Dack immediately dropped everything he was doing and ventured to Innistrad. He wasted no time in blundering directly into the vampires of the world, gaining the enmity of the Falkenrath in the process. However, what he could not know was that Sifa was far closer than he expected, and his actions had tipped her to his presence on the plane. To his horror he learned that Sifa forced a complete township of undead to attack, a tactic exactly like she once used in Drakeston. Dack managed to drive Sifa and her minions away, but he lost her before he could exact his revenge.
After this, Innistrad's secret guardian, Sorin Markov made his presence known. He judged that Dack had caused far too much of a stir in his short time on the plane. Sorin traded knowledge of Sifa's origins and Dack's life for the dagger.