Parry/Duck&Cover - Defensive Reaction: Player can use a reaction to increase AC by proficiency bonus vs one known attack per round

New combat rule: Parry/Duck&Cover - Defensive Reaction

Anyone can use their reaction to apply proficiency bonus to AC for ranged or melee attack they are aware of, if they declare it before the roll.   This is intended to add some realism and player choice when they are attacked.  The rationale is that in both fantasy literature and in historical martial arts combat, an aware person has some ability to react to a known attack by dodging, blocking, deflecting, or otherwise trying to avoid the danger.  The proficiency bonus grows slowly with level, increasing this benefit in a balanced way.  Against a missile, ranged or magic attack (dex-based save only), this reflects the instinctive tendency to duck or dodge when someone is aiming or throwing something at you.  Again, the attack must be known, meaning that Defensive Reaction can not be applied to sneak attacks or attacks that the player is not aware of.  If the reaction is used for defense, it can not be used for something else later or earlier in the round, like an attack of opportunity.   If the player is incapacitated or sleeping, or otherwise not aware of their soundings, they can not apply this reaction.  Optionally, the DM could allow this reaction to be applied to a saving throw vs an area of effect spell, such as a fireball, or cone of (whatever).  This would reflect the character seeing the spell caster targeting them, then intentionally "hitting the deck" or otherwise seeking cover from the spell.