D&D 5E Mass Combat Rules

I often find myself running large combats, both because my players love minions and because I find AD&D-sized monster mobs more aesthetic and believable than ubiquitous high-level monsters. (Plus, the jerk part of me loves the fact that PCs hardly get any experience from drow/hobgoblin mobs, in spite of their deadliness. Other times I give them easy XP encounters like Blue Slaads, which hopefully evens out my jerkiness with drow/hobgoblins. :))

Anyway, these rules are not about truly mass combats with actual armies, but they're rules that I developed after running a rather unsatisfying combat with 3 PCs, 1 spelljamming ship with 4 heavy weapons on it, 1 vampire, and 60 hobgoblin vampire (spawn) against 120 hobgoblins and an adult red dragon who was also a 12th level Sorcerer. This is for platoon- or company-level combat, neglecting morale effects, but intended to be more satisfying than straight-up applying the DMGs rules for mobs. It's based on the AD&D-era Spelljammer mass combat rules. I've used it both with grid/line combat and Theatre of the Mind.

Key insight: to make combat simple, you need to minimize the number of statistics you track. In particular, tracking HP for 120 to 200 monsters is hassle that you just don't need. This method is mostly about eliminating HP tracking while retaining things for players to do with themselves and their minions, especially die-rolling. If a monster has 39 HP, and an arrow does 1d8+1 damage (5.5 average), then it will take 7 to 8 hits on average to bring the monster down. If 40 archers are volleying, they can therefore expect to bring down a number of casualties this round equal to about 5 times their hit percentage, which is the same expected value as rolling 5 attacks against the monster's AC and making each hit kill a monster. Ergo, you can eliminate HP tracking by scaling down number of attacks without much loss of fidelity.

Procedure: Calculate average damage and average HP for each group of combatants in the combat. (For damage, I round up but ignore crits on the theory that they should roughly cancel out. So a 1d8 + 4 attack is counted as 9 points of damage.) Each round, each group rolls a number of "attacks" equal to the number of attackers times (target HP/attacker damage). Ignore initiative, all resolutions happen simultaneously. Every hit kills one combatant. Repeat for the next round.

Edge cases: You may have an awkward number of combatants that doesn't round well to a whole number. If you have 10 attackers doing 7 damage each to a group of targets that have 39 HP each, you don't want to roll only 1 attack per turn but neither do you want to round up to 2--neither one is fair. Instead, at the DM's option, you should roll 70/39 = 1.79 attacks per round. The easiest way to do this is to roll 2 attacks per turn. The first attack works normally (one hit, one kill) but the second one has only a 79% chance of a kill, which you roll on percentile dice.

You can apply this same procedure to any other awkward cases such as Fireball spells or attacks from weak PCs that can't quite finish off an enemy.

Fiction: The narrative fiction is that arrows are flying all over the place, the battlefield is in chaos, no one is keeping close track of who's already shot the guy you're Fireballing, etc. If a 100 HP bad guy goes down to a PC's 10 HP arrow (attack hits, and then the PC rolls his 10% chance of a kill), it must be because someone else already had weakened him. Conversely, if PCs keep hitting but missing their % kill chances, the attacks are still hitting--they're softening up enemies for others to kill but not quite bringing them all the way down. It tends to even out.

Exceptions: Either the attacker(s) or the target(s) of an attack can opt out of the mass combat rules for that attack. If so, revert to regular rules for that attack: roll (Speed Factor) initiative for that attack as normal, apply crits, and track HP damage instead of inflicting one-shot kills. In practice, that probably means that PCs will always be attacked normally (unless the players like to live dangerously), and only minions and enemy mobs will be attacked using mass combat rules.

Quick example (based on actual play, and using actual rolls): Four 7th level PCs and 11 skeletons are invading the ant hive from Quests of Doom/Of Ants and Men. Because their mission is to put an end to livestock deaths in the area, they've decided that they don't want to sneak through the ant hive and would rather kill all 800 ants in the hive. According to the module's tables, once the hive realizes that it is under attack, a combat will occur any time ants are encountered. In the upper levels, there is a roughly 50% chance of one to three worker ants or a drone showing up every minute, and roughly every five minutes there will be a larger battle group of up to 19 ants and/or 6 ankhegs or a bulette. A bulette encounter would be best dealt with using regular combat rules, but the others might be best suited to mass combat rules.

After killing the guards in the entrance, the PCs turn a corner in one tunnel and see 2 worker ants, looking agitated and aggressive by the distress phermones released by the dead guards. They are hostile. The DM has already calculated that it takes 4 skeleton hits to kill an ant, so the skeletons make 2.75 attacks this round. For the sake of the example I'll say three PCs are making 2 attacks per turn for 1d8 + 4 points of damage (one melee, two ranged) and the melee PC (a paladin) is on point; there's also a wizard in the back chucking cantrips for 2d8. 18/39 = 0.43 and 9/39 = 0.23, so the PCs have a 43% chance per hit to kill an ant while the wizard has a 23% chance.

Now the combat begins. The paladin has said that he wants to be attacked normally, so the DM rolls two attacks for the two worker ants. The paladin's AC is 21 so the ants need a 17 to hit. Rolls: 1 and 9, both miss. All the PCs have +7 to hit, and worker ant AC is 13, so they need a 6 to hit. Paladin rolls: 5, miss. Bard rolls: 11, hit. But 72 on percentile, so no kill. Monk rolls: 13, hit, 73 on percentile, no kill. Wizard rolls: 10 and 45, hit but no kill. The 11 skeletons need a 9 or better to hit, and roll: 17, 3, 9. One kill, and one 75% chance of a kill. They roll 65 and kill the second worker--the PCs did a good job of softening it up!

The PCs continue onward and spot another ant at 40'. It misses the paladin (natural 1). The paladins smites it to the earth (8 is a hit, and then 15 on percentile so it's a kill).

Everything is quiet for a few minutes and another couple of twists in the tunnel. Now the PCs see 6 worker ants, 5 soldier ants, and a giant bomber ant 34' away and coming towards them! The paladin says he's going to dodge, and the bard is going to cast Hypnotic Pattern as soon as they get close enough. Everyone else is going to attack as normal.

Only the workers are fast enough to reach and attack the paladin this turn, so the DM rolls for them: 12, 19, 1, 14, 2. The bard uses Cutting Words to reduce the 19 to a 17 (don't ask me how you insult an ant, but it obviously works), but it still hits the paladin for 9 points of damage. The bard also casts Hypnotic Pattern over all of the ants (they're small, and Hypnotic Pattern's cube is 51' from corner to corner, so it covers a large section of tunnel). They need to roll 14 or better to save: workers get 8, 10, 9, 5, 9, 20. Soldiers roll 15, 9, 16, 4, 4, 11, and the giant bomber ant rolls 12. Only 1 worker and 2 soldiers are unhypnotized. The paladin rolls 20 and 72 and wounds a worker, the monk rolls 3 and misses, wizard rolls 10 and 56 and wounds the worker some more, and the skeletons roll 1, 12, 20 (and then a 2 on percentile). The worker is down and so is one of the soldiers. (Soldiers have AC 14 and 45 HP, so skeletons can only make 2.44 attacks per turn against them, but it didn't make a difference in this case). Round one is over.

On round two, the soldier attacks the paladin: 5, a miss. Paladin misses (natural 1), monk rolls 13 and 35 for a kill on the last unhypnotized soldier. Then the bard wounds another soldier (18 and 79) while the skeletons kill two more soldiers and wound another (16, 13, and 12 + 78). Now there are 4 workers, 2 soldiers (one awake), and a giant bomber.

Play proceeds like that, and the battle is probably over in a couple of real-time minutes. Normally I might choose to skip over the end of a battle like this once the outcome is a foregone conclusion due to Hypnotic Pattern, but this style is fast enough that I think it's worth it to play it out for real in order to 1.) see if the paladin takes any more damage from the bomber ant or soldier ants, and 2.) keep the feel of being in the middle of a fast-paced ongoing invasion.

Conclusion: by eliminating HP tracking and most initiative rolls, I can play out large battles ten times faster than previously[1], and keep the focus on the PCs by minimizing the number of monster-on-monster actions without actually changing the balance of the combat. There's a tiny bit of up-front setup to calculate damage/HP fractions, but that only has to happen once. If you want to run large combats simply, and you're not satisfied with the way the Unearthed Arcana mass combat rules changes the time scale of combats, you should think about dropping HP tracking instead.

[1] And I can keep all the monster stats in my head instead of on paper indexed by monster number, which is a speedup in its own right as well as less mentally exhausting. "There's 4 soldiers and 2 workers still up" is so much easier than "worker 1 has taken 11 points of damage, worker 2 is undamaged, soldier 1 is down 23 HP, soldier 4 is down 7 HP, and soldiers 2 and 3 are undamaged."
 
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[Re-post from another thread, repeating here for reference]

I tried out another technique for large-scale enemies today and it worked pretty well, so I'll share it.

The PCs are trying to clear all the threats off a 25-square-mile rock island floating in the atmosphere of an air planet, so they can bring colonists in 4X-style. They used Commune With Nature to assess the local ecology, and learned that major predators include ropers, rocs, and phase spiders. This particular island had 90-odd ropers and 80-odd phase spiders scattered across various parts of its surface.

Now, I don't particularly want to have to play through 20-40 battles with phase spiders (I ruled that they come in groups of 1-6 spiders), so I made the players a deal: if you can fight a battle with disadvantage on every roll while the enemy gets advantage on every roll, we'll just call that your unluckiest battle of the day and say you won 9 other similar combats the same way. In other words, kill 2 phase spiders on hard mode and it will count for 20 spiders.

So we did, and it was a blast. First they located a smallish web in the forest canopy with two spiders in it and hovered the ship 120' over it, intending to blast them to death with catapults from above. They hit one spider and killed it, but the other spider won initiative next turn and vanished before anyone could hit it... and nothing happened for the next thirty seconds... when suddenly a phase spider materialized out of midair and bit the helmsman. Anyway, to make a long story short, they killed both spiders (20 spiders down! 14,000 XP!) and then went for their next in-game nest, which turned out to be 5 phase spiders. Now, I want to point out that 5 phase spiders is actually already a Deadly threat for an 11th level PC, a 5th level PC, and a 4th level PC (7000 XP, Deadly threshold is 5200) even before you factor in "disadvantage on every roll, and double-disadvantage on normally-disadvantaged rolls, and advantage for all your enemies". Before the encounter was over, the pilot had been knocked out and the whole ship was one initiative roll away from a 120' fall causing 12d6 damage to everybody aboard (undoubtedly would have killed the whole crew and probably all the PCs who were still up, and who by the way were still facing 4 phase spiders at this point), and there were multiple other close calls almost as bad. I had a blast and so did the players, and at the end we declared it the "worst battle of the day" and I handwaved another 45 spiders, so they killed 70 phase spiders today (in game time, I ruled that it took about two days of hunting) and earned 49,000 XP.

And then in the middle of their long rest, I spent some of my hoarded karma to open an interdimensional portal in the ship's cargo hold. The first thing that came out was a Nycaloth, and the second thing out was a Fireball which roasted the Nycaloth and all of the PCs' remaining cows that they use to fuel the spelljamming ship, so they're now stranded until they can find more lifeforms to put in the lifejammer. The third thing out was a Grey Slaad who was fighting with the Nycaloth for inscrutable Slaad reasons ("Slay evil!").

Forgive the digression.

Anyway, the "fight battle at penalty and I'll handwave X number of similar encounters" was successful IMO and I will probably use it again in the future. If you can kill 5 vampires on your worst day, you can probably kill arbitrary numbers of vampires on an average day.
 

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