The Eye of Vecna and the Hand of Vecna are two of the most powerful magical items in the history of Dungeons & Dragons, so long as a character is willing to risk their soul in the process of using them. In the current edition of D&D, a character needs to attune to a magical item through a period of meditation, and there is a strict limit to how many can be equipped. The limbs of Vecna require a little more commitment than one of D&D's standard magic items, as the player needs to physically remove one of their limbs and replace it with Vecna's former body part, in order to utilize their power.
In the lore of Dungeons & Dragons, Vecna was an incredibly powerful lich, who had ambitions of attaining godhood. These ambitions were delayed, when Vecna's most trusted lieutenant, the vampire Kas, betrayed Vecna and removed one of his hands and eyeballs. The power of Vecna was such that his discarded limbs maintained much of the power of their original master. Those who use the Hand or the Eye of Vecna gain incredible powers, but their soul is slowly devoured by Vecna's presence until they become nothing more than another one of his aspects. Vecna eventually ascended to godhood and he is currently the god of secrets in the Greyhawk campaign setting, but his name is feared throughout the D&D multiverse, and his limbs still appear from time to time, causing havoc whenever they appear.
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Vecna is one of Dungeons & Dragons' most feared villains and it's unlikely that players will ever get the chance to take him on in a battle. There's a better chance that they will stumble across one of Vecna's former limbs, and face the question of whether to use their incredible
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