Simple Monsters for Simple Combat

Here's the pitch:

• Select a damage die from {d4, d6, d8, d10, d12, d20}.
• Hit points equals maximum damage.
• Select a subtractive armor from {-0, -1, -2, -3}.
  

Throw it into whatever. The original idea to define monsters by die type is by my friend Emmy Verte of Spooky Action at a Distance, who was going to do something like that for her FLEE ruleset but decided not to (I think hers would've used contested rolls?). The idea always stuck with me, though, so just today I did the math with some additional rules and it works out really well.

I simulated 5000 against of each die type against each other, for each armor type. Below are the results: each row is the attacker, and each column is the defender in the same order (d4, d6, d8, d10, d12, d20).

  
Matrix for armor 0:
d4: [2, 3, 4, 4, 5, 8]
d6: [2, 2, 3, 3, 4, 6]
d8: [1, 2, 2, 3, 3, 5]
d10: [1, 2, 2, 2, 3, 4]
d12: [1, 1, 2, 2, 2, 4]
d20: [1, 1, 1, 2, 2, 3]

Matrix for armor 1:
d4: [3, 4, 6, 7, 8, 14]
d6: [2, 3, 4, 5, 5, 9]
d8: [2, 2, 3, 3, 4, 6]
d10: [2, 2, 2, 3, 3, 5]
d12: [1, 2, 2, 2, 3, 4]
d20: [1, 1, 2, 2, 2, 3]

Matrix for armor 2:
d4: [6, 8, 11, 14, 17, 27]
d6: [3, 4, 5, 7, 8, 13]
d8: [2, 3, 4, 4, 5, 8]
d10: [2, 2, 3, 3, 4, 6]
d12: [2, 2, 2, 3, 3, 5]
d20: [1, 1, 2, 2, 2, 3]

Matrix for armor 3:
d4: [16, 24, 32, 40, 48, 80]
d6: [5, 7, 9, 11, 13, 21]
d8: [3, 4, 5, 6, 7, 11]
d10: [2, 3, 4, 4, 5, 8]
d12: [2, 2, 3, 3, 4, 6]
d20: [1, 2, 2, 2, 2, 3]
  

At zero armor, we see it tends to take about two hits in order to defeat an enemy of equal capability (e.g. d4 versus 4 HP). This is to be expected since that number is 1 less than the average of two dice rolls (e.g. 2d6 is 7). Something cooler is that it takes quite a couple hits to defeat a high-armor but low-HP opponent, just because of how many attacks miss—but it's not like a crazy amount of attacks, either. Aren't stonger monsters kinda nerfed, as far as longevity goes? Yeah, but the idea is then characters don't have to be defined by number go up.

What you can do is divide these numbers by how many characters are in your party. For example, for a party of 4 with d6 damage each, it's going to take about 5 rounds to defeat a d20 monster with armor 3. Meanwhile, the monster is going to be able to basically pick one off per turn. You know what that means. Strategy! It's an awful idea to attempt that head-on. Maybe you need like a big siege weapon to make up the difference.

Possible monster tiers? Plug in armor values as you need. I'd say fellkin have armor 1, undead have armor 2, and dragons have armor 3. The below equivalences to HD aren't mathematical as much as like thematic.

• d4 (~1/2 HD): Goblin, skeleton, baby dragon.
• d6 (~1 HD): Orc, zombie, young dragon.
• d8 (~2 HD): Gnoll, wight, adolescent dragon.
• d10 (~4 HD): Ogre, wraith, mature dragon.
• d12 (~8 HD): Troll, spectre, old dragon.
• d20 (~12 HD): Giant,  vampire, ancient dragon.
  

Flesh & Grit?

The math is basically what Into the Odd and its descendants do, except that characters also take ability score damage after losing HP, and also there isn't really a unified scheme for monsters except some broad guidelines offered by Chris M about which number does what. So you could use the math above to figure out how long a monster will last, in terms of both hit protection and STR points (a.k.a. grit and flesh).

One easy way to implement this is to say each monster has extra HP (or 'grit') equal to a die roll of their type, in addition to their base HP (or 'flesh'), so our goblin has 4 + d4 HP. Below is what the simulation looks like for that; they can take quite a few more hits, but without much bloat or complication:

  
Matrix for armor 0:
d4: [3, 4, 5, 7, 8, 13]
d6: [2, 3, 4, 5, 6, 9]
d8: [2, 3, 3, 4, 5, 7]
d10: [2, 2, 3, 3, 4, 6]
d12: [2, 2, 2, 3, 3, 5]
d20: [1, 2, 2, 2, 2, 3]

Matrix for armor -1:
d4: [5, 7, 9, 11, 13, 21]
d6: [3, 4, 5, 7, 8, 13]
d8: [2, 3, 4, 5, 6, 9]
d10: [2, 3, 3, 4, 5, 7]
d12: [2, 2, 3, 3, 4, 6]
d20: [1, 2, 2, 2, 3, 4]

Matrix for armor -2:
d4: [9, 13, 17, 21, 25, 41]
d6: [4, 6, 8, 10, 12, 19]
d8: [3, 4, 5, 7, 8, 12]
d10: [2, 3, 4, 5, 6, 9]
d12: [2, 3, 3, 4, 5, 7]
d20: [2, 2, 2, 2, 3, 4]

Matrix for armor -3:
d4: [26, 38, 50, 62, 74, 122]
d6: [7, 10, 13, 16, 19, 31]
d8: [4, 6, 7, 9, 11, 17]
d10: [3, 4, 5, 6, 7, 12]
d12: [2, 3, 4, 5, 6, 9]
d20: [2, 2, 2, 3, 3, 5]
  

Below is the average grit, flesh, and total HP for each die size. By the way, we don't have to actually have different pools of grit and flesh; you could just define total HP as max plus the die roll.

Die Size	Flesh	Grit	Total HP
d4	4	2	6
d6	6	3	9
d8	8	4	12
d10	10	5	15
d12	12	6	18
d20	20	10	30

Closing Comments?

You could differentiate between attack capability and hit points, and you can use the tables above to figure that out as well—but honestly, I like the symmetry for how easy it is to remember. Why say that a monster has like 12 HP and d8 damage? Why not just 10 HP (plus grit maybe), d10 damage, and maybe -1 armor for some extra damage soak? Here's something cute: read "d" in dX as "danger". Spooky!

Part of my intent with this is to separate combat from ability scores/bonuses, but what if a monster needs to roll that d20? I think this is another argument in favor of grit as determined above, since you can use the same number as a bonus on a d20 roll (whenever necessary). This means that monsters of the same type have similar but different bonuses, without (again) extra bloat or complexity. So a goblin with grit 1 has 5 total HP, and adds +1 to d20 checks; whereas a goblin with grit 4 has 8 total HP and adds +4.

Finally, I think there's some room for 'bloodied' condition in here -- either defined as losing 1/2 total HP (easy with even-numbered maximums without flesh/grit), or as losing all grit and beginning to lose flesh instead (thematic!). Could be interesting.

See my previous post about calculating character longevity with armor class, for comparison!

One last thing: I've just published a review of Luke Gearing's The Country on Bones of Contention. Hope y'all find it interesting! :) It's a fantastic campaign premise.