I found this house rule at the same place I found “Shields Shall Be Splintered!”. So, once again, this isn’t mine. Maybe one day I will find where I got it from and credit it properly. If anyone happens to know, drop me a line and I will add a link.
Armour is great! It also gets battered and, from time to time, breaks. Your AC works just as described in the Player’s Handbook, meaning that armour usually makes you harder to hit, but there is an additional rule.
Every time you take damage from a critical hit (usually a natural 20 on the attack roll), your Armour Class is reduced by 1. The armour has been damaged, and it ablates. If the armour’s base value is reduced to 10, then the armour is ruined and must be discarded. Thus, for example, a heroine wearing a chain shirt (13 + Dex modifier) can suffer three critical hits before her armour is ruined. She still gets to add her Dexterity modifier to her AC, but the armour is of no further use.
Armour that is not ruined can be repaired. Assuming you can find a suitably trained armourer, the GM will decide what the cost of such armoury services might be.
One thing I don’t like about this rule is it has the potential to tax players without any payoff. With “Shields shall be Splintered!” the player gets to decide whether they sacrifice their shield. Which is part of what makes that rule so good. This becomes a much bigger deal to players when magic items are involved. (They tend to get attached to such things.)
I don’t want to create conflict between myself and the players. I want to create conflict between the characters, the NPC’s, and the environment.
With that in mind, magical armour is not destroyed outside of unusual circumstances. It becomes battered, broken, and can lose all of its natural protection (i.e., three points of AC for the chain shirt listed above). However, the armour retains its magical properties.
Lastly, what this rule ends up meaning to me. It forces the characters to seek out the services they need and build relationships in the campaign world that let them continue the fight. Or, maybe they will just learn to appreciate having the Smith’s Tools proficiency.