How to design a combat encounter for 20 characters? [duplicate]

Question

Over the course of the last 30 month, I GMed ongoing campaigns for three different groups. One of them had to be disbanded due to one problematic player. Another one dissolved because we couldn’t find enough time to play together. But I really liked some of the players (and their characters) in these two groups. So I integrated them all in the third campaign, which is still going strong. Everybody loved it. Some former PCs were now downgraded to NPCs, but my players like to take turns and play them during combat encounter. This big pool of characters (inlcuding support characters) could split up and tackle different obstacles within the campaign. It’s like Avengers: Infinity War, when everybody is in the same movie and cool stuff is happening all over the place.

Now, the end is near. The last session lies ahead. And my problem is, that there will be way too many characters running around in the last battle. I asked my players how they picture this confrontation in their heads and they answered: “We want an epic battle with ALL of our allies against the BBEG and his underlings”. I’m not a huge fan of encounters with too many combatants, but if my players want that, why not. But I have no idea how to pull that off.

Just to give you an overview. There are a Lvl9 Sorcerer/Warlock, Lvl9 Fighter/Wizard, Lvl8 Rogue, Lvl6 Druid, Lvl6 Artificer, Lvl6 Barbarian/Bard, Lvl6 Bard/Rogue, two Lvl6 Wizards, Lvl6 Paladin, Lvl6 Fighter, Lvl5 Wizard, Lvl6 Cleric, Lvl5 Fighter/Rogue/Bloodhunter, Lvl4 Fighter/Barbarian, Lvl4 Druid, Lvl4 Cleric/Artificer, a CR 3 construct, a ghost and a sentient familiar with kickass magic items.

That’s 20 characters.

In the last couple of sessions, I really challenged them with some encounters that had a CR way too high for their level. But they (ab)used some synergies between their characters and managed to win anyway. It was awesome. Now I’m afraid that the final encounter against the BBEG will either end as a picnic in the park, or with a TPK in round 2.

At first, I thought about pitting them against 20 enemies. This way I could make them roughly equal in power level to the party. But this doesn’t work for two reasons. For one thing, there is no reason why the BBEG would suddenly have so many previously unknown henchmen. He has some of his high-ranking generals by his side, all of which are known (and hated) by the player characters. I cannot justify ten new guys, because my players would ask “Who are they? And why do they show up in the last session and were never seen before?”. Secondly, if there are 40 individuals in initiative order, it will take an hour for 1 turn.

So I figured, if I drastically reduce the number of enemies, the encounter will be much more manageable. I just have to make the enemies more powerful. But as I’ve already learned, action economy is something to pay attention to in 5e. If my players focus on one enemy with their attacks/spells/abilities, that enemy is probably dead before his/her second action. No problem … I just make my enemies REALLY powerful. But that would also suck, because really powerful enemies would oneshot the player characters.

So my problem boils down to this: if I make a single enemy powerful enough to survive focus fire from 20 player characters for 1 turn, this enemy would also be powerful enough to kill one of the player characters in 1 turn. On the other hand, I could just quadruple the HP of the enemies and leave the other stats untouched. But then my players would ask why some of these enemies have 400+ HP for no apparent reason. So ...

How can I design a balanced combat encounter for 20 characters? If that’s impossible, what other options do I have to handle this problem?

Answer 1

Break it into multiple sectors

When working with large groups (especially groups of substantially different levels), each encounter should be subdivided or the waiting time becomes excessive for the players. This requires a bit of work and is better managed with the help of a co-GMPC for each group

What this solves

There are three problems with massive multilevel groups:

1. Time between turns
   • This can lead to boredom or a lack of flow
   • Having sectors of a fight reduces this effect.
2. Monster threat level
   • High power monsters will rapidly defeat the weaker characters while low power monsters will get killed too quickly by the stronger ones
   • You can balance around each sector separately.
3. Focus fire
   • This one you addressed
   • While I usually allow for some cross-sector influence, it is primarily independent sectors

In practice

1. Break the party into groups of approximate party level.
   • For your party I recommend the 8s and 9s, two groups of the 6s, and the 5s and 4s.
2. Create an encounter for each group
   • Balance this for the group itself. I'll address cross-sector influence in a bit.
3. Nominate a GMPC for each sector you won't run.
   • Sometime before each session, instruct the GMPC on the sector he is responsible for. Give then statblocks for the monsters and describe how the multi-sector encounter will work.
4. Break into separate tables and run the sector.
   • Each sector is treated as independent with one exception. See below.
5. Cross-sector aid
   • There is a Variant action a character can take where it aids another sector in the fight. This involves sending a message of aid to the GMPC or GM at the other table that aid was received. The next PC's turn (or current turn if it is already a PC) gets either advantage on their next ability check or attack roll (like the Help action) or can end one effect that an action from another party member could end (such as dousing a fire from a Fire Elemental). The PC receiving the help decides which to use.
   • Usually in the aid message I'll include the party member (character name and class) that is aiding so that the GMPC can use it in the narrative if possible.
   • If a table finishes early raise a "group finished" message to another table based on which group the finished sector wants to help. This essentially means that every party member gets the aid each turn.
6. Lair actions
   • If any monster in a sector has Lair actions, each sector in the lair will be affected. I usually decide about 6 or 7 rounds in advance (and tell the GMPCs) so that the Lair Actions are consistent across sectors.
7. Bringing it back in
   • Once every table is finished, the whole group can come back together. I often give a brief discussion so people can talk about any cool developments after the fight and then continue the story until the next encounter (designed the same way). With big groups, I just stand up front so people don't have to switch chairs.

Sample encounter

Obviously you can mix up the party how you see fit, but here's an example. Since full cross-sector aid becomes an option when one group wins their sector, I make the encounters a tad more difficult than usual, but this is a preference thing.

8s and 9s sector: They fight the demons that came out of the portal.

• 2x Barlgura [Monster Manual]

Lvl6 Druid, Artificer, Bards, and Paladin sector: They fight the warlock who created the portal.

• 1x Diviner (but with Warlock spells similar to Warlock of the Fiend) [both Volo's Guide to Monsters]

Lvl6 Wizards, Fighter, Cleric sector: They fight the cultists in service of the warlock.

• 3x Cult Fanatic, 1x Cult Leader (use Knight but with Cantrip High Elf feature and high Intelligence) [both Monster Manual]

4s and 5s sector: They fight the guards the warlock and cult hired.

• 3x Berserker [Monster Manual]