SRD:Horn of Goodness/Evil
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Horn of Goodness/Evil: This trumpet adapts itself to its owner, so it produces either a good or an evil effect depending on the owner’s alignment. If the owner is neither good nor evil, the horn has no power whatsoever. If he is good, then blowing the horn has the effect of a magic circle against evil. If he is evil, then blowing the horn has the effect of a magic circle against good. In either case, this ward lasts for 1 hour. The horn can be blown once per day.