The Fairborn is a magical knife passed down through the Edge family bloodline and named after it's creator.
It falls into the possession of the Hunters after they captured and tortured Axel Edge. Axel managed to break free and killed himself with the blade. In Half Bad Nathan Byrn, Gabriel Boutin, and Rose steal the blade but Nathan loses it while he is hallucinating and it is retrieved by Nesbitt, who in turn gives it to Van. Van eventually give's it to Nathan in Half Wild, Nathan uses the blade throughout the rest of the series.
The Fairborn is described as alive and thirsting for blood, whenever it's drawn the user is drawn towards cutting and attacking anything they can. It's shown to be very powerful as it was able to cut through a magical wall believed to be impenetrable. It can only be unsheathed by members of the Edge bloodline but once unsheathed can be wielded by anyone.
The Fairborn's design seems to be very simple. The blade is described as a deep black and not glossy or reflective at all.