As previously noted, incredibly heroic actions can sometimes imbue weapons, armor, or other items with magical power, granting bonuses or abilities to their users. Other magical items have been upgraded by less fantastic (though still magical) means; +1 shields, swords of life-drinking, and all manner of scrolls and potions are available, at a price, enchanted by mages and their hard-working assistants. Finally, some items are called from the ether by a Wish or a Miracle, often with a sole purpose in mind: slaying a dragon, capturing a demon, or driving back some monstrous force.
However, not all enchantments are, shall we say, beneficial. Occasionally, when trying to make a sword, a mage or cleric makes a mistake, and the result is a cursed weapon, or at best an item at cross-purposes to the original intent. Other items are cursed on purpose by mad mages or crafty clerics, and sprinkled liberally throughout their lair or hideout. That talking crossbow might be a +1, but it also never seems to shut up. Ever. And you're beginning to understand why it calls itself a Crassbow, too. And be careful of any gleaming blade found in a dungeon; while it may well be the Crystal Sword of Zefram the Immortal... who's to say his blade wasn't cursed? He was, after all, somewhat less immortal than one could be initially lead to believe. Cursed weapons often look amazing, but when it comes down to it, they fail - slipping out of the wielder's hand, never landing a blow, or even attracting arrows instead of repelling them. And regardless of effect, they entice their user, sparkling just so to draw his eye away from their obvious flaws; when snared by a curse, the user believes it to be a boon.
But those magical items are merely enchanted; an enchantment gone wrong is a pity, to be sure, but cannot hold a candle to the terrible items summoned by wishes gone wrong. Long ago, a man - not Zefram, though it likely wouldn't have worked out for him, either, as Zefram's luck had long ago been pulled into the red - wished for immortality. A simple wish, yes? A circlet of shining gold drifted from the heavens, and fastened itself about his neck... then drew tighter, and tighter, until he could neither speak nor breathe. And yet, he never died; unable to quench his thirst, or sate his hunger, or even draw a ragged breath to fill his aching lungs, he lived in perpetual torment, undying, yet hardly living. Only after years of suffering, long after his mind had snapped, was he granted an end to his immortality. Perhaps more gruesome, though with less suffering, is the story of a woman who desired to be desired. She wished for a brooch of seduction; perhaps not the best choice of wish, but usually safe enough. Except that a more powerful tool of seduction has never been found, for its power did not stop with man, or, worryingly, woman. Or worse, beast. Or even the living. Or indeed even the animate! Luckily, her life ended at the collapse of her own ceiling, desperate for her touch, rather than at the unwelcome touch of the horde of creatures, living and dead, swarming towards her door. Wishes are dangerous things; use them wisely, if at all.
Finally, some items of indescribable power have been imbued with the dark touch of their former masters. Twisted and bent by untold evil, these items go beyond harming their bearer; rather than allowing swords to more easily find their mark, they turn brother against brother. More than reducing the strength or charm of a handsome knight, they transform him into a hideous beast. Rather than destroy a man, they bring entire kingdoms to ruin. These chaotic items seek new masters to corrupt, shrieking in unholy rapture when they turn a good man to evil. Unlike items cursed by man or misfortune, these items are truly evil, and struggle towards the destruction of civilization.