The Ring of Damage permanently doubles all of your damage. It can be obtained directly with Diamonds in the Magic Shop or as part of the Starter Pack in the Magic Shop. Once obtained, your Heroine will always have this item.
Forged by the Dark Runesmiths in Muz Baluh, this ring permanently doubles all damage inflicted by its wearer.
If you have the diamonds to spare, this is a great item. But it doesn't give you anything you can't get by just putting more time into the game. Though at some point, it will be so expensive to increase your damage in small increments, that automatically doubling everything will be very tempting.