You got your GURPS in my D&D!

This obviously strikes a chord with this GURPS (and D&D) player, and I replied:

Even if you run screaming from the game, the implications of negated attacks and armor as damage reduction/resistance are fully fleshed out in GURPS. It assumes that an attack “good enough to hit” is only the first step, and there are two different opportunities to negate it – a defense roll and the “damage soak” provided by armor.

Lots of concepts implicit in the rules that you could choose to ignore or map to D&D mechanics.

In fact, I think I’m going to yoink this thread and see what I can make of it. :-)

Rather than write a post that says “do this,” I’m going to start with thinking about the kinds of things that might have to be true in order to map a GURPS-like combat sequence to D&D mechanics.

Why Bother?


Well, firstly, I obviously like the GURPS sequence of attack-defend-penetrate armor-resolve injury. I feel that it involves more player agency, since the defense roll also comes with a plethora of tactical options, including yielding ground, special parry types, damaging parries, and the ability to do a “riposte” that sacrifices the ability to defend this around for an extra increase to hit in a following round. 

So yeah: if you just like roll 1d20+bonus vs. your AC, by all means keep doing it. I do it five times a month and have a riotously good time, so this is in the nature of a thought experiment.

The Key Questions


GURPS asks different questions for resolving attacks than does D&D. They are, basically

1) Did you throw a blow good enough to hit a target, assuming he doesn’t do anything about it?

This seems like a no-brainer, but it’s not. If you’ve even been in martial arts training, you’ve either thrown, or seen thrown, kicks and punches that are terribly, awfully mistimed, or (more often) where the distance is just completely wrong. The defender could just stand there, and the attack would still miss. In fact, more advanced students will do exactly that, while beginners will attempt to defend anyway. 

In GURPS, the basic hit chances can start out pretty low, especially for Joe Normal. A punch would default to DX, while a weapon attack would probably default to something like DX-5, which is a crazy-low Skill-5 which you can see in one of my more widely-read Melee Academy posts is really, really awful. If you’re attacking another Joe Average, you’re probably looking at an All-Out Telegraphic attack. That would be an 83% chance to throw a blow worthy of landing (but your foe’s defenses will be 50% or higher in that situation). 

But I digress. The key is, the first question is “did you throw something worthy of hitting a doofus who’s basically just standing there?” It doesn’t take that many points in skill, plus the All-Out and Telegraphic options, to make the answer to this question “Yes, yes I did” often enough that you can presume it. 

Of course, you can’t defend if you do that.

2) OK, here comes a blow worthy of hitting. Does the foe defend, and how?

This one gives you three options. You may always try and dodge; and if terrain and your maneuver selection allows, you can also retreat for a big honkin’ bonus. If Joe Untrained can back the hell up, this one will start at about 62% chance of success, boosted to 83% if he’s receiving a Telegraphic Attack.

He can also parry, which is to use a weapon or unarmed technique to ward off the blow. This is based on combat skill, and the more skilled you are, the better your defenses. But it takes a +2 in your skill to give you a +1 to defend – it’s presumed to be harder than attacking.

Finally, if he’s got a shield, he can block. This is basically a parry with a shield, but it also works with arrows (which normal parries do not) and has some advantages when parrying weapons like flails. 

Dodge is based on your speed, equal to 3+(DX+HT)/4; Parry and Block are 3+Skill/2.

3) OK, you hit the guy. Was he wearing armor? If so, did you hit it hard enough to either penetrate it or deliver damage through it anyway?


Once you hit and your foe fails to defend, you roll damage . . . and if your target has no Damage Resistance, he takes HP of injury. If he does, you subtract the DR from the damage. This can nullify the attack, even if it hits.

4. You penetrated his armor. Is he dead yet?


This one’s pretty universal. Get down to 0 HP or lower, and Bad Things happen.

Good grief, get to the D&D part already!


Right. Now we start to play.

The D&D question set is smaller. It basically treats steps 2 and 3 as a single, passive score. If you overcome this score, you proceed to 4. Now, there are a few exceptions. You can Dodge in D&D, a whole-turn action that, well . . . the rules are now online. So:

DODGEWhen you take the Dodge action, you focus entirely on avoiding attacks. Until the start of your next turn, any attack roll made against you has disadvantage if you can see the attacker, and you make Dexterity saving throws with advantage. You lose this benefit if you are incapacitated (as explained in appendix A) or if your speed drops to 0.

With apologies to WotC, I’ve linked some commentary I made about Advantage and Disadvantage, a mechanic that keeps on giving. Really, it’s genius.

Still, what happens here is you roll your attack, and if you beat the Armor Class of your foe, you injure him – or if you’re bugged about injury in the face of short rests, you at least reduce his Hit Points.

A starting D&D character – who is probably NOT Joe Normal – swinging at a guy with average stats in mail armor (call it a chain shirt) will roll 1d20+2+his STR bonus, and that’s likely to be a +3 if you choose from the Basic Array and play a human. So 1d20+5 vs an AC 15 (assuming DX bonus of +2, from the Standard Array). Basically a 50% chance of doing injury.

It of course will depend on how far you want to go with this, but in general, if you’re going to look at attack, defend, absorb damage:

• Hitting should probably be easier, and you should get better at it as your level increases. The second part (thanks to proficiency bonuses) is true already.
• Defending should be a thing. Dodge and defensive movement might still be rolled into a passive effect, or they might be active effects. 
• Damage resistance would have to be worked out by armor type. Weapon damage might need to increase to compensate. Maybe not. If high level means you are negating more and more attacks with active defenses, this may mean HP need to come down. Maybe way down.

Let’s Try


OK, so we’re going to GURPSify D&D. [Cue howls of outrage. OK, better now? Good.]

The attack roll

I’m tempted to just say Roll 1d20 plus the usual bonuses vs. a DC of 10. This gives our Joe Average (well, not exactly average, if he’s a 1st level fighter with STR 16, CON 15, DEX 14, INT 11, WIS 13, CHA 9) at first level 1d20+5, and a 20th-level character with STR 20 and a proficiency bonus of +6 a 1d20+11. He’s always going to hit. As he should.

I’m going to speculate that we’ll want how well he hits to matter. In GURPS, this is done by the mechanic of Deceptive Attack – you take a penalty to your hit roll, and half that penalty applies to your foe’s defenses.

This is a bit more risky than the Margin of Success method, but this is D&D, not GURPS – let’s forget that. We’ll go with a single roll, which determines your quality of hit:

Make an attack roll vs. DC 10. Note your margin of success.


Level 1 character: average hit chance 75%; average margin of success on a hit 7.5.
Level 17 character (assumes STR 20): 95% hit chance; average margin on a hit 12


The Defense Roll


The defender gets a roll to ward the blow. The skill of the character (or level of the monster) should matter for parries and blocks. Many animals and monsters will simply try and dodge. Let’s call that Evade, to distinguish it from the official Dodge rule above.

Evade


This should probably be a DEX-based roll, against something like 8 or 10 plus the foe’s DEX bonus, and maybe the proficiency bonus as well. At 1st level, that’s going to be about +4, while the incoming hit roll will have succeeded (or else you wouldn’t defend) and so have a margin from 0 to 15 (assuming another 1st level assailant). If you want two first level characters to stay more or less the same chance of a successful blow landing, defenses are going to be pretty low. Something like only succeeding 30-35% of the time. So if you’re rolling with DEX and proficiency of +4, you’re looking at DC 18 or so, which conveniently means your target might be something like 10+Margin.

How does that work for our Level 17 hero? A fighter gets seven ability score increases, each of which is a +2. He can get to his STR 20 with a two +2 bonuses, or a single +2 and two well-chosen Feats. That leaves four or five others. Let’s assume he gets a single +2 to DEX, with another +2 for CON, and then two or three actual martial Feats. So STR 20, DEX 16, CON 18, and a bunch of Feats, probably four (one of which probably raises STR by 1). Lots of ways to get there, but the point is, our Level 17 fighter is rolling 1d20+9 against a DC 22 incoming blow. He’ll succeed 40% of the time vs. a foe of his own quality, and against the 1st level guy at DC 17, 60% of the time. 

I don’t think this is enough disparity between Level 1 and Level 17 here. But then, our Level 1 character will be rolling 1d20+5 against our Level 17’s AC of 20 assuming non-magical plate and a shield. 30% chance to hit. Against the active defense roll, he’ll make a successful attack 75% of the time, and Level 17 will fail to defend 40% of the time . . . for a 30% chance to hit. Maybe not so bad after all.

Block

This is just a defense using a shield. Again, skill matters, so proficiency counts. I’m tempted by four options here:

1. The shield’s usual bonus to AC of +2 adds to the roll, making it 1d20+Proficiency+2 (Shield Bonus)
2. Double the shield’s usual bonus to the roll: 1d20+Proficiency+4 (1d20+6 for Level 1)
3. You get your STR bonus plus the shield bonus. For our +3 STR guy, that’s 1d20+7 (shield, STR, proficiency).
4. You get your DEX bonus (retaining DEX as the thing that makes you harder to hit with armor), proficiency, and another 2 for the shield. Our sample Level 1 guy is 1d20+6 in this case, picking up 2 for each.

The DC of the incoming attack doesn’t change – about 17 for the Level 1 attacker and 22 for the Level 17 one. In theory, you want about the same as dodge, but maybe a little better. So I’ll pick option 4, and retain DX.

Parry


Again, this one is going to be similar, with proficiency counting to your ability to parry. However, for this one, I’m sorely tempted to allow STR to be the dominant factor here, since it’s your STR that gives you bonuses to hit when attacking, and so perhaps it should also give bonuses to parry.

That would make our Level 1 guy parry (with STR 16) at 1d20+5. That means his best defense would be a block if he carries a shield, second best is a parry, and third is dodge. Not unintuitive for a STR-based fighter.

Damage Resistance


A bog-standard longsword will do 1d8+2 in one hand for our level 1 guy, and 1d10+2 in two hands. If that has to (say) punch through armor before it does injury . . . well, that’s a whole ‘nother ball of wax.

We’re not going crazy here – no calculations of armor thickness. I’d just start with the native AC of the armor – 10. So hide will only remove one point of damage from an attack, while full plate will provide 8 points of protection.

Clearly, this begs for a modifier to the hit roll to aim for chinks in armor, which might (say) halve the damage resistance, rounded down.

The implication there, though, is that instead of doing 1d8+2 for 3-10 HP per hit, against our guy with the standard chain shirt (AC 13, or in this case 3 points of protection), he’ll do slightly less, 0-7 points, but 90% of his blows will still be telling. Against more serious armor, like plate, he’s looking at 0-2 HP of penetrating damage per hit, and 75% of his swings will be nullified.

I’m guessing you’d want to cut HP in half, roughly, to keep the fights from taking forever.

Parting Shot

Why would you ever do this rather than just play GURPS or some other system with active defenses? 

Well, for one, D&D-type games are the #1 force in the tabletop RPG market (though other kinds of games, like card games, are likely even bigger). So if you want a system nearly everyone plays, you’re into D&D, Pathfinder, and the OSR. 

The other reason, of course, is because it might be fun. One of the nice things about playing Dungeon Fantasy with GURPS is that you get more options on both attack and defense. The game is very interesting from a tactical perspective. You can go all-in on your attacks, sacrificing your defenses to try and strike home accurately. You can do the same thing and trade off defenses for a really hard hit. You can aim for various hit locations. You can retreat bit by bit and hold a foe at bay with a long-reach weapon.

Some of these things you can do in D&D, but many you cannot. While the games I play in that are helmed by +Erik Tenkar and +Ken H are outstanding fun, I do miss some of the cool things I could choose to do with GURPS that take combat beyond “I hit him with my sword again for 8HP more damage.”

I also really like not just sitting there when attacked. Yeah, it’ll slow down the game by making every contest two or three times as long from a rolling dice perspective. Every time the GM or player rolls a notional hit, you have to defend (or not – there needs to be an option and a benefit to not defending), then roll damage, subtract armor DR. I’m used to that in GURPS, and I feel it enhances my game experience rather than detracting from it. 

Personally, I really like the agency. I also like that the quality of the hit in the concept presented above matters. The better you roll, the harder it is for the foe to defend. 

Obviously it would need tons of testing – but I started out wondering if you can map the attack-defend-damage paradigm onto D&D, and whether or not it’s a good idea, it seems plausible and not inherently game-breaking off the bat.

One-Step Opposed Resolution


I had a funny feeling this would be true, and it is. My reliance on 10 as a base DC for both attack and defend allows me to write the following:

Hit occurs: 1d20+Attack Bonuses > 10, or 1d20+Attack Bonuses – 10 > 0

Defense Successful: 1d20+Defense Bonuses > 10 + 1d20+Attack Bonuses -10

Defense Successful: 1d20+Defense Bonuses > 1d20 + Attack Bonuses

No surprise there. It’s a contest. The only caveat is if your attack or defense roll inlcuding the bonus is less than 10 (the base DC for most of this stuff), you fail anyway. 

So the sequence, without the math, would simply be:

Attacker rolls his to-hit roll: 1d20+Bonuses; Defender simultaneously rolls his chosen defense: 1d20+Bonuses. 

• If Attack roll < 10, you miss.
• If Attack roll < Defense roll, you miss.
• If Attack roll > 10, defense roll is <10, you hit
• If attack roll > 10, Attack Roll > Defense Roll, you hit

This combines well with +Peter V. Dell’Orto‘s idea in the comments, since Dodge gives advantage (roll twice, pick the best) to the Defender, while an All-Out attack would give advantage to the attacker, and rather than have them somehow cancel out, you can just use the rules independently.