Sword of Obsidian

The Sword of Obsidian is an epic two-handed blade sold by the Blacksmith for 20 obsidian ore. This sword is quite powerful, due to its large damage output and the multiple bonuses it procures. To wield this sword, you must wear the whole obsidian set.

Source

 * Blacksmith

Bug
You can wield this sword even without the obsidian set. (This was apparently patched, you need the armor set to use the sword)