Diablo/Anvil of Fury

At caves level 10, go back to town and talk with Griswold. He will say something about a magic anvil that contains demonkind magic on it, however, that was just a legend that was told to him. But he will ask you to check it, and, if is true, maybe he can get something for you as a reward.

Backtrack to level 10 and search for the anvil, it will be somewhere here. When you find it, back to town and hand it to Griswold. He will be amazed, the legend was true. As a reward for you, he will make you - using the anvil's magic - the unique item: Griswold's Edge, that is very powerful.