I've always liked this rule, as it meant I didn't have to worry too much if the group got their hands on their enemies items. They may enjoy a temporary (and sometimes necessary) power bump, but nothing was permanent. I also loved how it fit in with the idea of the dwaer being creatures of darkness; creatures so wicked that even their gear was bound to the darkness of the deep realms and perished once exposed to the cleansing light.
In one of my current games, the group have gotten embroiled in all kinds of Dwaer'Syth stuff, and have obtained a few choice items (spoilers: one item is cursed, but has yet to reveal its nature) . These "Dwaercraft" items are quite potent, but are destined, now they are away from the subtle power of the underdark, to begin to slowly unravel. Yes, even the cursed one.
Here are my rules for just this. I hope they are useful to someone.
Dwaercraft items are crafted in eerie forges of silence and shifting arcane pressures. Dwaer smiths use strange alloys and dark magics to bind the unnatural radiations of their home realm to these items - energies that rapidly dissipate when the item is removed from the depths for at least 10 consecutive days (5 if exposed to sunlight for even an instant).
Once such items have begun to decay, nothing short of a Wish can stop it, though certain other items have been created that delay or prevent this happening..
Once an item begins to decay, it loses one point of bonus every 10 days. Once the bonus reaches zero, the item crumbles into strands of inert, useless ash. Items that do not have "plusses" decay dependent on their rarity. Common items Rot after 5 days, uncommon 10, Rare 20, Very Rare 30 and Legendary 60.
To.combat this decay, various individuals have crafted items over the years that delay or prevent it. Here are two examples.
Dark Charged Resonant Shard (Very Rare Wondrous Item) These shards are imbued with the essence of the deep realms, and can be slotted into a Dwaercraft item to prevent it decaying. They are slender, twisted flecks of darkly iridescent, glassy material that subtly crawl with dark energy. One slotted (usually in a slip silver or permanent slot worked into the item) to an iten it becomes immune to the decay associated with being removed from the deep realms and / or exposed to sunlight.
Night Infusion Oil (uncommon consumable) An application of this darkly luminous oil covers one medium sized weapon or carried item. Two are needed for a large weapon or suit of medium armour. It takes 1 minute to use, but once applied delays the onset of decay caused by being removed from the deep realms or by exposure to such light for 1d4+1 days.
Nightbond Oil (Very Rare consumable) An application of this shimmering, volatile fluid grants a single weapon or set of armour a permanent +1 bonus (max enchantment +4). However, that item becomes a Dwaercraft item, and has all the weaknesses common to such items.