A blade forged of the Eternal Night
Shadowblades can appear as any manner of bladed weapon, and can vary from wispy and ethereal to glossy and solid as steel. They are in fact a manifestation of shadow magic drawn from Mabar, the plane of Eternal Night.
The first documented shadowblade was discovered by an adventuring company contracted by the government of Breland to venture into the Mournland after an artifact. While there they were attacked by an adversary who wielded one. After the enemy was slain, one of the adventurers discovered on the corpse what appeared to be a glossy black shard of stone which radiated shadow magic. After further examination, he discovered that it was a living piece of shadow in and of itself, and that it could be used as a focus for shadow magic in order to manifest a weapon that could survive only briefly away from its creator’s grasp.
Other rumors of shadowblades had existed in Xen’drik for ages, with claims that a tribe of Drow used them in battle, but the one brought out of the Mournland is the first documented confirmation of the existence of these weapons. However if was a little while later than their true nature was discovered. A party of adventurers exploring deep under Stormreach, discovered a manifestation zone that was coterminous with Mabar, and that the magic of shadowblades was drawn directly from that plane.
It has also become clear that without great power, the use of a shadowblade had unexpected effects on the wielder, with the power of Mabar slowly growing on the user’s mind, and in some cases taking control or driving the user mad. This knowledge has since led most people to avoid the use of such weapons, though some have experimented with using less potent forms of shadow magic to create similar weapons.
Encyclopaedia Eberronica, Sixth Edition. ed. Rauthur.