The Beastslayer Blade was a powerful weapon specially forged to slay the Alien Beasts which plagued northern Valorim. Most likely still in the possession of the Surface Explorers or their descendants.
The Beastslayer is said to have been the joint creation of three Dragons: Athron, Khoth and Sulfras. Although they did not care about the Plagues which affected the Empire they did find the Alien Beasts an annoyance. So to jointly rid themselves of the Alien Beasts and prove their non involvement to the Surface Explorers and Avernum; the Beastslayer was forged. Athron provided the Mithral it was created from, Khoth provided the enchantments and Sulfras herself forged the blade.