Excalibur

Excalibur is a blade forged with dragon's breath fire, and another undisclosed metal. This weapon has a curse applied to it. The curse makes everyone it impales to die within the time of the battle. If you get a minor scratch, only your lifespan decreases. The sword's effects can be nullified if you have tough enough armor or shield to make the sword do absolutely nothing.
For the effects to work, you need to hit the opponent's actual body. If the opponent makes a physical clone of themselves, you have to hit the actual body, not the clone, for the effect of the blade to take place.
Altair and Fors both wield the Excalibur despite the fact there can only be one.
Message from Altair: In most other timelines, I loved you. It seems this one had to be the one with the spite.