I always liked the Dark Sun interpretation of shadow as something separate from darkness, and dependent on light. The novels have various shadow creatures that are almost unstoppable in direct sunlight, but if you can get them indoors into darkness they become far weaker.
That goes against the way Shadow tends to work in 3e and 4e though. In the Forgotten Realms books, there are a series of feats aligning you with the Shadow Weave as opposed to the regular Weave. These feats make you better at spells with the Darkness descriptor, as well as Enchantment, Illusion, and Necromancy. They make you worse at Evocation and Transmutation spells (unless they have the Darkness descriptor), and totally incapable of casting spells with the Light descriptor.
So the obvious way of having an anti-Shadow artifact would be to load it up with Light effects. Spells like searing light and sunburst would fit the bill. 2e (and likely 1e) had some cool moon-themed priest spells, like starshine and moonbeam that might fit the bill as "light out of shadow"-type spells (although, those spells tended to be pretty weak for their level).
A more subtle approach would be to attack the Enchantment/Illusion aspects of Shadow Magic. Effects that give you clarity of thought and vision. True seeing and mind blank might be a bit too absolute, but maybe a more specific dispel illusion type of effect.
