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?