The appearance of an artifact or relic must always be the basis of an adventure. These items should never be casually introduced into play.
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Artifacts and relics always possess dangerous and possible deadly side effects. These effects are all but irreversible, unaffected by wishes and most greater powers. Artifacts can only be destroyed by extraordinary means.
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These unique objects were once held and used by gods and mortals far greater and more powerful than normal men. Often these items existed for an express purpose - to be used by a particular hero, to fight a particular foe. So closely associated is an artifact with a person, time, or place that its powers can seldom be fully used except by specific individuals who meet certain standards. A weakling could not hurl Thor's hammer, nor could just anyone command Baba Yaga's hut. An artifact may show its full powers only to deal with particular, very specific, threats or dangers. Artifacts have purposes, sometimes fulfilled long in the past and sometimes never-ending.
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Once the adventure is over, it is best for you to find some way to get the artifact out of the players' hands. In essence, the artifact was a MacGuffin - the ting that made the plot go - not something you want to remain in your campaign now that the need for the item is gone. This is very much in keeping with the nature of artifacts and relics, since they have a maddening habit of disappearing once their task is done. To leave the artifact in the campaign is to invite abuse by the player characters, perhaps for noble ends, but abuse all the same. There are, even in a fantasy game, "some things man was not meant to know."