The Adamant Sword Seth takes to Zzyzx was given to him by Newel and Doren, who salvaged it from the tar pit where Kurisock lived. The sword is described to be tempered adamant, possibly even stronger than regular adamant. The scabbard is encrusted with jewels and was crafted by the Nipsies.
When retrieving Vasilis from within the Totem Wall, Seth is instructed to leave behind the sword and his adamant shield as collateral. At Zzyzx, Bracken says that the blade will pierce most greater demon hides.