ACTIVE DEFENCE

This is a follow up to my post about alternative armour systems for OSR games, which can be found here: https://aloneinthelabyrinth.blogspot.com/2020/01/80s-armour-two-alternative-armour.html

This post concerns replacing enemy hit rolls with PC defence rolls, and some other variable rules to either expedite combat, or add additional flavour without layering on too much simulationist crunch. I was first exposed to this idea while looking into u/giffyglyph’s Darker Dungeons O5R homebrew document but have since discovered that it’s a very common house rule, especially in GLOG circles including the excellent Die Trying by Spwack.

"Wow!" indeed... courtesy British Library

This represents my own interpretation of similar houserules.

THE BASICS

DM declares that a player is being attacked (either in melee or from range)

• The player rolls a d20. They add their defence bonus to this number.
• If they roll equal to or higher than their opponent’s attack, they have successfully defended against the attack. 
• If they roll equal to or less than the attack number, they have been hit and the DM rolls damage as appropriate.

Defence Bonus

This is calculated in the following manner:

• Ascending AC: AC -10.
• Descending AC: 9 -AC.

So a character in leather armour holding a normal shield with average dexterity will have a defence bonus of +3. (AC 13 -10/ 9 -6 AC)

Attack

Calculating the attack score of enemies can be slightly more complicated, depending on the system used. Typically, it works like this:

• OD&D, B/X and BECMI clones: 10 + monster’s hit dice* + any attack bonuses listed
• AD&D 2e: 30 - THAC0 
• Later Editions: 10 + monster’s listed attack bonus

*Note hit dice is rounded up for the purpose of calculating attack: a 1+1 HD monster adds +2 to their attack.

Using this method, a typical goblin should have an attack of 11, meaning an unarmoured, average character with no shield has a 50% chance of defending the attack, which is in accordance with the goblin’s 50% chance of hitting them. 

(they'll be a bit beefier if you're converting from 5e, and you'll probably want to make a separate ranged attack value for them, too)

Making the Roll

An unmodified roll of 1 is always considered a failure, no matter how  puny the incoming attack and how mighty the PC’s defence: this is considered to be a critical hit on the part of the attacker.

An unmodified  roll of 20 represents excellent defending on the part of the PC: the DM may decide to award some kind of bonus such as a free attack or advantage on their next attack etc. 

Ah stuff it, I’m going to make a table:

Critical Defensive Success! For when you roll a natural 20...
Roll 1d6	This Happens
1	You make eye contact with your opponent for a second. Use this opportunity to wink at them, or try to say something witty.
2	You make eye contact with your opponent for a second. Use this opportunity to wink at them, or try to say something witty. Their morale and that of their comrades drops by one (if applicable, otherwise no effect)
3	Your opponent makes themselves vulnerable: you receive +1 if you attack them at the next opportunity. Alternatively, you may disengage and flee without any penalty.
4	Attack of opportunity! You may immediately make a free attack against your fumbling foe.
5	Disarmed! Your opponent drops their weapon, if applicable. If they are unarmed, see 4, above.
6	Disarmed and vulnerable! You manage to disarm and get the chance for a free attack! If they are not carrying a weapon, you may  gain +2 on that attack.

This is just for inspiration, yeah?

ADDING VARIETY

With the players now visualising combat in terms of their characters defending against incoming blows, this may give you the opportunity to introduce other minor rules variants to make combat a little spicier.

Fighting Defensively

Instead of making an attack, PCs may elect to defend against any incoming attacks using whatever weapons or shields they possess. If unarmed, they may instead try to dodge, parry or otherwise evade their opponent’s strikes.

• Roll 2d20 for their defence roll, keeping the highest score and then adding their defence bonus. 

(5e players will recognise this as being the descendent of the dodge action.)

Attacks from Behind

A standard rule in OD&D (and some variants of later editions) was that shields do not provide any bonus to AC if an attack comes from behind. Different editions vary as to whether this also affects dexterity bonuses.

• Remove dex and shield bonuses from AC if characters are attacked from behind.

The upshot of this is that it gives everybody something akin to a thief's backstab/ sneak attack ability, and will encourage some sneaky play.

• Remove the normal "to hit bonus"As an option, make every strike a crit from behind, or crit on any roll of twenty, including modified rolls.
• As an option, a DM may allow the player to modify their defence roll against attacks from behind by their wisdom bonus, to represent the character's awareness of their surroundings.

All of these options could also apply to defending against attacks while surprised.

Defending Against Multiple attackers

D&D usually lets the law of averages take care of defending against multiple attackers: the fact that the PC is being mobbed is represented by the probability that at least one of the strikes is going to hit.

• For added realism (and to encourage a bit of tactical thinking), ganging up on an opponent could penalise their defence roll by one for each additional opponent.
• From the PCs perspective, each additional character attacking a single opponent will get an equivalent bonus on their attack roll: the first additional attacker gets +1, the second +2 etc. 
• If used in conjunction with defensive fighting,  the first attack is defended with two dice, taking the best roll.The second attack is defended normally. It is not until the third attack that the PC is penalised.

Defending with and without Weapons

• A character who is unarmed and is defending themselves against an armed character suffers a -1 penalty. No such penalties apply if the character is armed.
• A PC armed with a sword or quarterstaff (or other weapon the GM deems appropriate) receives an additional bonus to their attack roll.

Attack Bonus as Defence Bonus

If characters gain a bonus to their attack roll as they level, consider applying this same bonus to their defence rolls, too: as they get better at fighting, they get better at defending as well as attacking.

More Shields

The standard OD&D shield modifies AC by one, and by extension modifies defence by one. As an alternative, consider different sized shields:

AC of Shields
Shield	Defence/ AC Modifier	Cost	Encumbrance	Hit points
Small/ buckler	+/- 1	5	5 lbs	5
Medium	+/- 2	10	10lbs	15
Large  (Kite shield etc.)*	+/- 3	15	20 lbs	20
Tower Shield**	+/- 4	25	45 lbs	25

* PCs with large shields cannot receive more than a +2 to their defence due to their dexterity.
** PCs with tower shields cannot use their dexterity to improve their defence. On the flip side, low dex characters are no longer impeded.

Hit points are for PCs who sunder their shield: 

• if they are hit by a powerful blow, PCs may state that they are sundering their shield
• The shield is destroyed, absorbing the total hit points worth of damage.
• Any remaining damage is deducted from the PCs HP. Don't forget to modify their defence: they no longer have a shield.

Active Defence with Armour Rating Rules

For extra fun, combine the two rules for armour as damage reduction with the active defence rules.

• Armour no longer contributes to the PC's defence bonus, instead count dexterity modifier (if any), shields (if any) and attack bonus (if used).
• PCs will now be easier to hit but...
• ...they will possess an armour rating that may reduce the amount of damage they receive from an attack. 

Courtesy British Library

And so concludes my foray into the world of pen and paper combat vis-a-vis armour and defence. I'll be collating this into an easier to read and printer friendly PDF at some point, keep your eyes peeled! Additionally. let me know if you have any thoughts concerning these rules hacks and those described in the previous post 80s Armour: Alternative Armour Systems