Tenebris

"That's Tenebris, black as night and sharp as sin, forged from the Light-Bringer's wrath inside of the Morning Star's core. When he fall, the sword got stuck between a slab of obsidian, only to be drawn by him alone when the final battle begins. Oberon apparently heard of this story, got inspired by it and decided to do the same thing with Caledfwlch. He think it's funny if the Holy Sword that choose the rightful Christian King can only be drawn in the way the Devil himself is prophesized to draw his own sword. The angels and demons, however, didn't think so."

- Matt Wright.

Tenebris is the Nephalem sword that is wielded by the fallen angel-turned-Demon Lord Lucifer.

Overview
The name 'Tenebris' is Latin for 'darkness' which fits Lucifer's title as the Prince of Darkness. Tenebris is a weapon created by Lucifer himself using the energies of both Hell and Heaven, and was also used to battle his twin brother, Michael.

After Lucifer's fall, the sword fell afterwards, and is said to have plunged in the depths of Hell unto a rock of brimstone where it is unable to be picked up by any demon or even ones of high power and authority like the princes of Hell or Archdemons.

This is evidence of the fact that the sword itself acts as though it were alive and conscious, and therefore, answers only to its one true master, Lucifer.

With Tenebris, Lucifer is able to channel his power through the blade itself, allowing him to perform wide ranged attacks and long ranged as well. The sword also gives off a specific colorful glow when it becomes imbued with Lucifer's power. Example being a bright gold when he accesses his Light abilities, and a shadowy black/smoke when he accesses his Darkness abilities.

Quotes

 * "Tenebris is the name of Lucifer's sword and is made from the most ancient of darkness. Legends said that when said "Let there be light", he created Lucifer and when he divided the darkness and the light, he forged the darkness into the sword and gifted to the Archangel as the first birthday gift in existence." - Carl Black.

Image Sources

 * (Tenebris.jpg) Dark Sword by ???
 * (Tenebris the Dark Sword.jpg) Tenebris the Dark Sword by ???