I was thinking object rules with disadvantage if wielded too.The DMG has rules for breaking objects. The AC in those rules assume you easily strike and instead determine whether you do significant damage rather (it says so), kind of like heavy armor. However, with an attended object it ought to be harder to pull off. 3e would have you make multiple rolls to handle that mess, but 5e doesn’t do that. If you want to feel like 5e, you need a ruling that doesn’t add rolls. Since the AC number actually matters (it’s what separates adamantine from wood, for example) you shouldn’t ignore it.
5e would probably suggest attacking an object according to the DMG rules, but applying disadvantage when the object is worn or carried.
It’s also worth noting that adamantine weapons now have the same property as Shatterspike according to Xanathar’s.
In my own conversion of this adventure a while back, this is what I came up with for Shatterspike, since there's no standard "sunder" option.
Shatterspike (Rare). Taken from a dead paladin in the Sunless Citadel, Shatterspike is an heirloom of another age. It a +1 longsword, and on a hit, the wielder can choose to break an item instead of dealing damage. On a critical hit, the wielder can choose to both deal damage and break an item. The item must be held or obviously worn. Armor and large pieces of clothing cannot be targeted in this way. A non-magical item or a common magic item is automatically destroyed. A magic item that is Uncommon or higher gets a DC 10 Charisma save to resist. If it is not intelligent, it uses its wielder's Charisma bonus (if positive). Rare items get +2 to this save, Very Rare get +4, Legendary get +6, and Artifacts are immune.
It worked out quite well. One player kept it through level 20, ultimately exchanging it for a scimitar of speed found in Nightfang Spire, but keeping it for utility purposes like smashing locks. It was a lot of fun.
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