The problem with parry rules is that the abstract d20 attack roll is supposed to include multiple attacks and parries. So, the question becomes: how do we make a player's "I parry!" statement meaningful, without changing the meaning of abstract combat as it stands?
I tried doing this before by making "I parry!" a declaration of intent to all-out parry. Forget that. It's bleh.
Instead, here's a new idea:
- Only one parry per round --- the called parry --- has special benefit, and only against one opponent's attack with a hand-to-hand weapon (or bite/claw/stinger.)
- A called parry aborts any non-combat actions, like casting a spell, or any combat action that doesn't involve using hand-to-hand weapons, like firing arrows. If you've already taken a special action like that, you can't call for a parry.
- If you use a called parry against an opponent, that attack roll must be higher than your Dex as well as higher than their normal target number. If not, the attack is parried.
- Any other attacks against you made after the called parry are treated as always. They can be parried, blocked with a shield, dodged... whatever.
And, to re-iterate, you can still attack with a mêlée weapon. Also, I'd allow sacrificing a weapon (having it knocked from your hand, or in some cases broken) to avoid damage, just like in "Shields Shall Be Splintered".
The advantage of this rule is that it doesn't add any extra die rolls, is easy to remember, and quick to resolve.