This nasty weapon was rare until the Accordance War; it is designed for assassination, and as such has been officially proscribed. Naturally, enterprising Unseelie nockers have taken advantage of this to raise their prices.
The dagger has a small compartment in the hilt. When a part of the intended victim is placed in the compartment (a strand of hair, an eyelash, or a drop of blood), the blade becomes attuned to that person, and can never be reattuned. Attacks against that target are made at a -3 difficulty to hit, and do an automatic extra die of damage. Attacks against anyone else are made as a normal weapon. Once the target has been killed, the blade begins to corrode, disappearing in less than an hour. Though daggers are the most common of this sort of weapon, other types are possible (a bane arrow would be particularly nasty). Because potent magic is required to make these weapons, bane daggers cannot be made of Cold Iron.