D&D 5E Swarms in 5E

[Dausuul]

I tended to envisage the phalanxes in question as containing 20 to 50 hobgoblins, depending on size (huge or gargantuan). When I used them, they could regen hp by soaking a nearby hobgoblin minion; I know others have used an approach where on its "death" the swarm triggers the appearance of a handful of minions in its space.

The benefit of using a swarm rather than 20 to 50 hobgoblin minions is that it makes managing the round, modelling force multiplier effects, etc much easier. For instance, I can just give the phalanx a "Phalanx rush" power to enable it to make a Reach 3 close blast attack with push that also lets it shift up to 3 sq into the area of the blast. This is much easier to adjudicate than rolling 20 hobgoblin attacks, deciding which ones "aid another", etc etc.

I don't dispute that there is value in having "mass enemy mechanics." However, there is a fundamental difference between using shortcuts to quickly handle 20-50 foes, and modeling foes that number ten thousand, or a hundred thousand, or millions. With 20-50 hobgoblins, a fighter swinging a sword could chop them all down in a reasonable amount of time. With the wasps, that just won't happen. That's why I regard the wasp swarm as a "terrain hazard" situation; it is not appropriate to expect the party to kill all the wasps. If you want, you can burn limited resources to disable the hazard; a mid-level spell or some flasks of alchemist's fire. Or the heavily armored fighter can charge through and get whatever it is you're after on the other side. Or you can come up with a creative solution like flooding the room. But this is a situation in which simple combat--the fighter swinging a sword and the wizard shooting off attack cantrips--should not be a viable solution.

And even with the 20-50 enemies scenario, I don't like the 4E swarm approach, mostly because I dislike mechanics that radically change the flow of combat depending on how you choose to model the enemy. Let's say the PCs find themselves facing 50 hobgoblins. Being moderately high-level, they decide they can take 'em all in a straight-up fight. The party wizard opens with a fireball, which hits about half of the area the hobgoblins occupy. What happens?

If the DM decided to model the hobgoblins as individuals (minions, or mooks with fewer hit points than fireball damage on a save), 25 of them are now dead. That means they take up 50% less space and have 50% less damage potential. Furthermore, the particular area where the fireball was dropped is now clear of hobgoblins, with the tactical implications of that. The party can rush into that space and seize an advantageous position. And the hobgoblins must now decide: Close with the PCs and hope the wizard won't bomb her own party--at risk of having their entire force wiped out if the wizard decides the other PCs can take the heat? Or scatter and try to take down the wizard with ranged attacks, at the cost of sacrificing their phalanx bonuses and melee strength?

If the DM decided to model the hobgoblins as a swarm, none of this happens. The swarm takes a painful hit, thanks to its AoE vulnerability, but it's still dishing out just as much damage as before and taking up just as much space. There's no question of scattering to use ranged attacks, either. There is no advantage in doing so--the swarm is just as vulnerable to the next blasting spell whether it stays at range or closes to melee.

For handling a situation like this, my preference would be a system to quickly resolve large numbers of attacks and special abilities, where you can plug in "I have X monsters with +Y to hit, attacking a character with armor class Z. Without having to roll every single attack, here is a result that approximates the outcome." Tracking damage to the hobgoblins is seldom necessary. If the PCs are high enough level to go head to head with 50 hobgoblins, it's a safe bet that all their attacks are lethal enough to kill a hobgoblin in one hit.

 

[Dausuul]

For modeling a horde of monsters, here's a solution that roughly approximates fighting individual monsters:

HORDE: Sometimes there are so many monsters that tracking them individually becomes ineffective. In this case, the DM may wish to treat the monsters as a horde.

Horde Statistics: The horde has two special stats: its AC threshold and its damage threshold. AC threshold equals 11 plus the attack bonus of a member of the horde. Damage threshold equals the maximum hit points of a member of the horde.

Melee Attacks: A PC in melee with the horde suffers 1d6 hits per round. Each hit deals average damage for a member of the horde. For each of the following, subtract 1 from the number of hits:

• Every 2 points by which the PC's Armor Class exceeds the horde's AC threshold.
• The PC has at least one adjacent ally.
• The PC's back is to a wall.

A roll of 6 on the die always inflicts at least one hit.

Ranged Attacks: A PC subject to ranged attacks from the horde suffers 1d6-1 hits per round for every 5 horde members targeting that PC. For each of the following, subtract 1 from each die roll:

• Every 2 points by which the PC's Armor Class exceeds the horde's AC threshold.
• The PC is lightly obscured (by smoke, dim light, etc.).

A roll of 6 on the die always inflicts at least one hit.

Damaging the Horde: Any time a PC deals damage to a member of the horde, that member dies if the damage equals or exceeds the horde's damage threshold. Otherwise, the horde takes a casualty token. If the horde already has a casualty token, discard it and the targeted creature is slain.

Area Attacks and the Horde: If the horde is the target of an area attack, use the following as a guideline for the number of members hit. The horde tends to "clump up" around enemies, so if any PCs are in melee with the horde, add +5 targets for each PC in the blast radius, up to a maximum of twice the base value.

• 5-foot radius: 2 targets.
• 10-foot radius: 6 targets.
• 15-foot radius: 15 targets.
• 20-foot radius: 20 targets.

Once the size of a horde falls below 10 members, the DM should switch to handling them individually. If the horde has a casualty token when this happens, one of the members is at half hit points.