This is one of 5 cursed items you will find on mobs in each of 5 different tombs set up in a circular pattern in Ancient Necropolis.  Take all five of the cursed items and swim to the bottom of Soundless Lake, directly in the center.  At the bottom of the lake, you find a large circular alter-type structure named Soundless Lake Sealstone. Use the cursed items while down there and it will purify all five of the them. Once purified, head back to the large central structure in Ancient Necropolis and use the anvil. This will consume all five purified items and give you a Royal Tomb Key.  You should be able to use on any of the 5 doors within the tombs.