Durza's sword was a sword of unspecified type and size.
The blade was "long, pale, with a wire-thin scratch winding down it". The scratch had been inflicted by Ajihad in a duel with Durza some years ago. The sword must have been incredibly durable to withstand the force a Shade could put into a blow. It was not, however, equal of the Riders' swords forged by Rhunön, as those, among other properties they might or might not have shared with this blade, could never be scratched. The blade was also relatively narrow, narrow enough to slip between ribs and strengthened by the energy of the Spirits in Carsaib, this was mentioned by Oromis. On one occasion, an angered Durza threw the blade at a tree, which was harmless, even though it had been thrown with enough force to pass halfway through.
No specifics are given as to when, where or how Durza came by his sword. Its earliest known appearance in Alagaësia's history was in the duel between Durza and Ajihad, which took place at an unspecified time before the events of Eragon, but presumably not before Ajihad joined the Varden some sixteen years earlier. Most likely the sword has been forged by Durza himself using his black arts.
Durza is first mentioned wielding his sword in the duel with Ajihad some sixteen years ago. Later, he had it with him in his ambush of Arya in a mission to capture Saphira's egg. When Eragon rescued Arya from Durza's clutches in Gil'ead some months later, he encountered the Shade for the first time, along with his blade. They later clashed their weapons at the Battle under Farthen Dûr, where Eragon destroyed Durza once and for all, but not before receiving a grievous slashing wound across his back from the sword. It is unknown whether it was the property of the sword to bestow the curse Eragon suffered from greatly during the following months (until his physical and mental change at the Agaetí Blödhren), or if this was rather the result of Durza wielding it and then cursing his wound with Dark magic. The fate of the sword is not revealed. It is not mentioned again and it is not known whether or not it survived Durza's demise. If it did, then it was probably destroyed or buried after the battle, as it is not likely that any of the Varden would be willing to use a Shade's sword, no matter how fine a weapon it might be.