The Fang of Haynekhtnamet is a literal fang which was carved into a blade and fashioned into a small dagger. It is named after Haynekhtnamet, a mythical wamasu from the Shadowfen region of Black Marsh. Scholars believe that Haynekhtnamet is not actually a single creature but a series of wamasu, and as such it's possible that there are multiple Fangs.
The most famous Fang of Haynekhtnamet was created by a score of Nords who fought Haynekhtnamet for seven days and nights before finally bringing the creature down, believing it to be a dragon. One of the survivors took a fang as a trophy, and it was later carved into a dagger which mysteriously houses some of the beast's magical properties. The dagger grants the wielder the ability to shock an opponent, and has been seen occasionally by traveling heroes throughout history. This particular Fang came into the possession of the Sixth House during the late Third Era. It was reportedly recovered by the Nerevarine and sold to the Mournhold Museum of Artifacts in 3E 427.
There is evidence of a second Fang of Haynekhtnamet, which was taken from a wamasu in 2E 582. Fashioned in an Argonian style, this jagged blade was heavily befouled and would cause disease in anyone it struck. 
- In ESO, this item is named "Swamp Raider's Fang of Haynekhtnamet" due to being part of the Swamp Raider set. Wearing several items from this set will fortify weapon damage and stamina, as well as increase the potency of poison and disease attacks.