Alternate Armor – Avoidance DCs

I know MCDM was prototyping it for a little bit, and the feedback was that it really felt like the GM was playing by a different set of rules than the PCs. I’m interested to see how it would play out – I made a rudimentary version of 5e where monster attack rolls were replaced with PC armor rolls, which works the same way.

Basic version:
Avoidance DC. Monster attack rolls turn into the Avoid (save/dodge) DCs of the attack. for easy math, it’s 10+attack modifier. So a +4 to hit would be a Avoid DC of 14.
Avoidance Bonus. PCs would find their Avoidance Bonus by taking their AC and subtracting 12 from it. So a character with 18 AC would get an Avoidance bonus of +6.
Crits and Crit Fails. A PC gets crit if they roll a nat 1, and automatically avoid the attack if they roll a nat 20.
PC Attacks. PCs attack the same way they usually do, versus the enemy’s AC. Only the Monster attacks are changed.
Advantage and Disadvantage. If an enemy has advantage against you, you roll Avoidance with disadvantage. If the enemy has Disadvantage, the PCs roll with Advantage.

So, let’s go with the following example (and proof of the math): An Owlbear is attacking a fighter with plate armor and a shield. The owlbear has a +7 to hit, and the fighter has a 20 AC. Normally, the Owlbear hits on a 13 or above, a total of 8/20 rolls, and fails on 12/20 rolls. If we use Avoidance instead, the Avoidance DC is 17 (10+7) and the Avoidance Bonus is +8 (20-12). The PC avoids the attack on a 9-20, a total of 12/20 rolls. They get hit on a 1-8, a total of 8/20 rolls. The math is the same!

I chose to make the PC modifier the awkward one since they only have to do the calculation when their AC would change, while the DM will have to do it just about every encounter.