A topic was recently started on the RPG.net forums about a magical weapon that would have the sundering ability. I ran with Illithidbix’s ideas of using a combination of the rust monster’s ability combined with the Disarm rules from page 271 of the DMG.
Sword of Sundering
Very rare magical weapon, requires attunement
This +2 longsword glows with a sickly green light. This sword was created to destroy it’s opponent’s defenses and ability to attack.
Any hit on an armored creature causes the the armor worn to take a permanent and cumulative -1 penalty to the AC it offers. Armor reduced to an AC of 10 is destroyed.
You can instead choose to target your opponent’s weapon or shield. You make an attack roll contested by the target’s Strength (Athletics) check or Dexterity (Acrobatics) check. If you win the contest, the attack causes normal damage, and damages your opponent’s weapon or shield.
Strikes against a shield cause a permanent and cumulative -1 penalty to the AC it offers. A shield that drops to a +0 bonus is destroyed. Weapons hit take a permanent and cumulative -1 penalty to damage rolls. If its penalty drops to -5, the weapon is destroyed.