Old School Mass Combat I - The basics (attacking, taking damage, and saving throws)

The idea started here but you can definitely skip the other posts. This is a straightforward rewrite that will be easier to understand, while the other posts were mostly brainstorming. 

My goal is creating a mass combat system that fulfills the following criteria as much as possible:

- Perfectly compatible with B/X*. 
- Minimal conversion needed.

- Avoid approximations. AC 4 and AC 5 are different things, and having 5 HP is better than 4.
- Results that are similar to individual combat.

- No new units needed, use ANY humanoid or monster in the book.
- Avoiding new mechanics, such as using 1d6 or 2d6 for attack rolls.

- Miniatures and maps are optional, you can go full "theater of the mind" if desired.

(*This also means it could be used with other OSR and TSR games with minimal conversion, for example, adding +1 to you attack bonus, etc. You can use it for Dark Fantasy Basic as written, using ascending AC).

Notice that most existing systems do not fulfill these criteria, including chainmail and other cool systems that use 2d6 attacks, 1d6 attacks or different armor values, for example. In my system, AC, damage, etc., is kept exactly the same.

First, we are proposing two systems. You can use either or both, depending on the circumstances. The first system (detailed) uses units of 20 individuals as a starting point and has very precise results when compared to individual attacks. The second system (freeform) uses any number of individuals. It is easier to use but the results do not map perfectly to individual attacks.

These two systems are for calculating damage output. Every other rule (taking damage, morale, fighting individual PCs and NPCs, etc.) applies equally no matter which system you choose.

TBH, as a fan of minimalism, I'm very close to just abandoning the detailed system altogether. Skip to the second one if you want.

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1a) The detailed system

Start with a of 20 NPCs per unit. When a unit attacks a monster or other units, it doesn't make an attack roll. Instead, it simply roll for damage. The damage caused by a score every turn is equal to the damage caused by an individual multiplied by [21-THAC0+AC].

Unit damage = single damage * [21-THAC0+AC]

An unit of veterans, for example, causes 1d8 * [2+AC]. Against a target with AC 4, they cause 6d8 damage per round.

If you're using ascending AC and attack bonus (AB) instead of THAC0, the formula is NPC DMG*(21+AB-AC). The results are exactly the same (e.g., the veterans cause 6d8 damage, etc.).

If you want units of 100, 200 or 1000, just multiply the damage by 5, 10 or 50 as appropriate.

Smaller units should divide damage as appropriate (e.g., an unit of 10 NPCs deals half damage - 3d8 in our example). One possibility could be just attacking as normal until you're reduced to 10 NPCS, and only then reduce damage by half.

Units of 13 or 17 are possible but the math gets harder: roll damage, divide by 20 and multiply by 13 or 17. You can use approximations if you want (e.g., an unit of 6 or 7 just deal one third of the original damage), move on to the freeform system, or fall back to individual attacks when there are few NPCs left.

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1b) The freeform system

In the freeform system, the damage of an unit is equal to the sum of all individual damage, averaged and divided by three. For an unit of 20 veterans, it would be 30, for example (20d8, average 90, divided by 3).

To hit, simply make three separate attack rolls. Each successful attack inflicts 30 points of damage. 

Three is an arbitrary number, of course - you might as well divide the total damage by ten or twenty and roll as many dice, but three attack are enough for me.

If you lose about a third of your unit and don't want to recalculate, you could reduce to two attacks instead. However, recalculating is very easy even without a calculator (17 veterans cause 17d8 damage, a bit less than 6d8 when divided by three, so let's say about 25 damage per attack) - or, if you prefer, calculate (or even roll 6d8) after you hit.

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2) Taking damage

When an unit takes damage, just divide it evenly to see how many NPCs are down. A group of 20 veterans (4 HP per NPC) taking 30 damage loses 7 individuals, excess damage is discarded.

Notice these individuals aren't necessarily dead, but they are out of the combat for all intents and purposes.

Single figures (e.g., a single dragon in the battlefield) loses HP as normal. If they survive, they might test morale, and they start the next round with fewer HP.

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3) Saving Throws

Use the same reasoning, rolling once for every third of your unit. Area damage might be treated differently; see "area attacks", to be resolved in another post.

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And... that's it. It is enough to create all kinds of mass combat. The rest are special circumstances that have to be treated separately, including: 

* Initiative

* Movement and attacks (charge, melee, ranged)

* PCs and powerful monsters

* Leaders

* Mounts

* Spells and other area attacks (a fireballs affect about a  dozen NPCs in formation?)

* Flanking

* Maximum number of attackers

* Prices

* Time (turns etc.)

* Fortifications

* Melee weapon reach

Also, the fact that I'm not using HD instead of HP and d6s instead the d20 might deserves an addendum.