The Stone of Zurak was forged by the gods during the Titan Rebellion before the rise of the current races. The God Zurak forged it from Pure Adamantite. He blessed it with his divine power. It takes an expert smith to be able to control metal being shaped on the anvil, however if they can bend the metal to their will the anvil automatically imbues a weapon with +1 magical power. In practice this means that any enchantments being made on a weapon are cast as if they were 1 lower than they normally are.
To be able to control the anvil a smith must make a Craft DC 25; the base DC for masterwork weapons is 20 and the anvil adds an additional 5 to the difficulty.