Long forgotten these blades were, forged to kill a powerful and ancient demon. When the deed was done, these blades were hidden away, for they carry a terrible curse themselves. The bearer of them has to kill someone for each day, for the blades hunger for blood. Failing to do so will turn the blades against the owner, literally sucking the life out of them.
In daylight, the blades look rather harmless, like a normal black blade, but during the night it will cast a ghostly red fire, harmless for its wielder, but burning for everyone and everything it touches. They also enhances the abilities of their current owner, making them almost invincible in every sword fight.
The blade has a life on its own, whispering to its owner trying them to do stuff they usually wouldn't do.
The curse of the demon blades cannot be broken. Once touched, the blades will be this persons curse - until the owner dies.