Using Magical Arms & Armor Mechanic From Chainmail In D&D

Instead of providing a bonus to the attack and damage rolls, magical weapons in Chainmail grant additional attack rolls per round. Conversely, magical armor negates a number of attacks per round.

The part that makes this feel properly magical is that an opponent can be reduced to zero attacks, and thus incapable of harming the wearer.

This can be used in Dungeons & Dragons almost as-is. The only question is whether the attack negation by armor is per round or per opponent. As magical weapons would only grant additional attacks per round, the complimentary rule for armor would be attack negation per round. But the magical bonus on Armor Class normally applies against every attack. If the attack negation is per opponent, a wearer could simply be invulnerable against a horde of low-level opponents.

Chainmail has a solution for this: opponents can utilize tactical teamwork which combines their number of attacks. This is superior to D&D mechanics like Help Action and Pack Tactics which would be insufficient against a per-opponent ruling.