Crystal of Vengeance

A crystal that can only be found in relics and weapons excavated from ancient battlefields. Old weapons not only retain the feelings of hatred and obsession of the people killed by them, but there is often a lot of residual magical power from their many battles. This crystal, which was created by these feelings and the magical power interacting, reacts to all sorts of lingering regrets, including the thoughts which collect in such objects. It is said that the more powerful the weapon, the more the crystal amplifies its power, and the brighter it shines.