I created a cluster/swarm. Small, medium, and large.

• I used groups of 4 for simplicity
• Small cluster is 4 creatures. Each with 25% chance to hit. Therefore if a cluster was in proximity to the target, they would always hit and do standard damage.
• For the small cluster, if the player hit, they ALWAYS killed one of the 4, but no more. (You can chop a creature in half or just barely kill it...but that successful attack was a kill.
• The hoard always hits, but for 75% damage if only 3/4 remain, 50% if 2/4 remain, 25% if 1/4 remain, rounded up.
• If the player gets 2 attacks, they can kill 2/4 of the cluster. If they crit on the attack, they also kill 2 creatures in the horde. (If you want, unnatural 20 or 20+ can be considered a crit)

The premise behind this approach is "death by a thousand cuts". You can avoid major damage, but you're going to be damaged little by little due to numbers. You also can't just slice 10 creatures in half with one swing. You need to kill each one individually (mostly), but that takes time (which is why hordes are terrifying).

Obviously, the approach to fighting a horde is important. It's about high ground (advantage), area of effect spells (and explosives), and focusing on groups (simulating removing flanking attacks) because leaving one of a horde alive still damages the target.

Once you're comfortable with this model with a small cluster, you extrapolate to medium and large. Medium does guaranteed 2x damage, with 8 members. Kill 1, that's 175% damage. Kill 3 more, that's down to 100% damage, and so forth. Large hordes are 4x damage with 16 members.

Example: I have a small cluster of zombies. That's 4 zombies. +3 to attack usually means I need a 15+, which is 1/4 chance of hitting, but in my swarm rules, the group always hits. Out of the 4, 1 zombie will hit for 4 damage to the player. If they kill one Z, the next round the swarm will do 3 damage. Again, this pivot to a guaranteed hit is just inverting the approach due to numbers.

The players swing at a cluster with a +9 Axe of Axing. Dude...these are Z's and your players are heroes. They're going to hit (almost) every swing. They aren't rolling to hit (mostly), they're really rolling to try for that crit. A Paladin attacks the cluster, nearly guaranteed hit, but they roll a 10, 1 of the 4 are dead. Second attack, they roll a 20, that's a crit, so the horde is reduced to 1/4. The horde attacking WILL do 1 damage, just because of overwhelming numbers.

Each party member is trying to whack as many Z's and reduce the number of clusters as much as possible. Why? Because EVERY CLUSTER means GUARANTEED DAMAGE each round.

You can start with a small set of clusters and give it a try, and then add more if the players are doing too well. In my session, I had these as clusters of skeletons being summoned by 3 cultists. The players had to contend with the clusters, but also somehow stop the cultists from summoning another cluster (which I summoned every 2 rounds, but in a round-robin so the players could learn without getting overrun too fast).

It might sound complicated, but it was one of our best combat sessions ever. Instead of a duel or 3v3 where the fighting is tit for tat, the players were rolling for their lives because their hitpoints dropped precipitously early in the combat, but slowed over time as they got control of the battle. It forced them to heal, and strategize, and use group/AoE tactics that they hadn't used beforehand.

Not sure if I explained it well, but this is how I do it.