Item from The Stormlight Archive by Brandon Sanderson
A sentient black sword from another world, inscribed 'Destroy Evil' — it devours Investiture ravenously and is possibly the most dangerous weapon in the cosmere.
Nightblood was created on Nalthis using a thousand Breaths and the command 'Destroy Evil' — but a sword can't understand evil, so it destroys everything it can reach and asks questions never. Unsheathed, it devours Stormlight, Voidlight, Lifelight, and the wielder's own soul if nothing else is available. It killed Rayse, the original Odium. Szeth now carries it, which is either the wisest or most terrifying decision anyone has made. Nightblood is chatty, cheerful, and completely unable to comprehend that it is horrifying. It considers itself a force for good. It asks its wielder if they'd like to destroy some evil today. It is a weapon designed to kill gods, carried by a man who was already the most dangerous assassin on Roshar.
A black-sheathed sword that leaks dark smoke when drawn. The blade itself is pure black, absorbing light. Black tendrils of consumed Investiture trail from it when active. The sheath suppresses its power, but even sheathed, Nightblood projects a compulsion — the worthy feel nausea, the evil feel irresistible attraction.
Also known as: Nightblood, the black sword