How much more dangerous is an encounter with the PCs outnumbered and how do I balance around it?

Question

I want to build an encounter after defeating the "boss" of the dungeon that the PCs are currently in, where the skeletons in the crypts they had previously passed through begin to rise from their graves. My goal is to make coming out of the dungeon a frantic race against time.

What I have so far is:

• Skeletons begin to animate, first the ones closer to the boss, next round some more further away, the round after a couple next to the door. That will happen in every room but I will keep a rough track of how much time they spend in each one to decide if the next one will be harder (if they stayed too long in the previous, skeletons would begin to spawn next room) or easier (if they're already having a hard time and not just delaying to get more exp).

• I will make abundantly clear the danger of not hurrying out, seeing as this is a crypt, if every skeleton in there comes back to life they are as good as dead.

• I'm thinking of borrowing the idea of Minions from 4e D&D and giving those skeletons 1hp to not make the encounter tedious and lend to the cinematic effect of "there isn't enough time, I can't afford to waste more than a swing on every enemy."

What my question is:

• I know that according to the DM Guide being outnumbered automatically raises the difficulty but how much?

  Put another way, how many 1hp skeletons can I can throw at a party of 4 PCs of the 2nd level (cleric, paladin, wizard and warlock) without them being entirely overwhelmed?

Answer 1

Alternative Solution: The enemies don't matter

You said that

My goal is to make coming out of the dungeon a frantic race against time.

Therefore, it doesn't really matter how many skeletons you pitch against them, as long as it feels like a battle they can't win. This is a place where the numbers are irrelevant and it's more about creating a compelling narrative.

_{Side note, if you haven't yet read The Angry GM's How to Build F$&%ing Awesome Encounters! (warnings for inappropriate language) you may find it an excellent thought exercise given that you say your group leans towards a roleplay-heavy style. I'm going to be using his guide for inspiration here.}

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The Mission

So you want to build an encounter. Before anything else, you're going to need a hook. Luckily yours is pretty easy to isolate; they've won their big baddie fight, and they now need to leave. Preferably alive.

The Mission: Get out.

But there's a complication; the longer they stay down there, the more skeletons come alive and try to murder them. It's tempting to say that our mission is then

Get out without being killed by skeletons.

But there's an interesting point to be made here; the danger they're facing isn't actually skeletons, it's time. It doesn't matter how many skeletons they do or don't fight, their chances of survival get slimmer the longer they stay in the crypts.

So let's revise our mission now that we know the danger:

The Mission: Get out quickly.

We also have the setting; a series of winding crypts that they may or may not remember how to get out of. They're going from Point A (the boss' chambers) to Point B (the exit) and need to do so as quickly and efficiently as possible.

To me, this no longer sounds like a fight encounter, because fighting isn't the mission here. Instead, I would treat this like a chase, which is going to play out very differently and likely feel a lot more urgent.

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Planning a Chase

The Angry GM does a much better job of explaining this in detail than I ever could, but here's my quick take on it. I've actually run two different "chase" scenes via this method and my players loved it. They were the kind of sessions they were still talking about next week, which is always a good sign!

1. Choose Decision Points

You know your players are going from Point A to Point B. Decide how many decision points you want between those two places. Each decision point is a place where your players can choose to do something that may help or hinder their escape.

Maybe there's a closed door they need to open, a pit they need to cross, a trap they need to disable, and a dark place where they can't see. You've now made a 4-decision-point trip from A to B. At each of these hurdles your players will need to make snap decisions as a team to get everyone through.

If you were to "map" this out it might look like

Boss - door - pit - trap - dark - Exit

Each of these points should have its own "skill challenge" applied to it. I'll explain what I mean by that later.

2. Add your baddies

There has to be a reason why they can't just take their time through your decisions, otherwise the chase loses all meaning. Luckily, you already have this reason; skeletons!

For the sake of this encounter, it seems logical that a party moving at average speed can cover the same amount of distance as an army of skeletons can. Therefore, both groups will progress at exactly the same speed, neither gaining nor losing ground, as long as nothing is done to accelerate or impede this progress.

This gives us a unit of time to work with; in one "round" the skeletons can move one "space".

3. Plan your skill challenges

You now know that the skeletons will progress one space per round. Now you can decide how many rounds your players should need per decision point, where you can allow them to gain or lose spaces, and how hot on their heels you want your skeletons to be.

This sounds like a lot to plan, so let me show you just one single decision point as an example:

At the start, your "map" looks like this:

hallway || hallway || hallway || locked door || hallway

Aka, there are three spaces of hallway before your locked door that your party needs to get around. Let's say the party has gotten to the door, and they can hear an encroaching skeleton hoard behind them.

(skeletons) hallway || hallway || hallway || (party) locked door || hallway

For each thing they try to get the door open, the skeletons will advance a space. If the skeletons get to the same space as the party is occupying, the party will die. (Or, if you don't want it to be a pass/fail mission, maybe they just need to fight a fairly hard encounter to win back ground and keep moving.)

Your rogue tries to pick it with lockpicks. He fails his skill check.

hallway || (skeletons) hallway || hallway || (party) locked door || hallway

Your wizard tries to burn it with magic. He fails his skill check.

hallway || hallway || (skeletons) hallway || (party) locked door || hallway

Your fighter tries to bash it down. She succeeds her skill check!

hallway || hallway || (skeletons) hallway || open door || (party) hallway

The party bursts forward into the hallway as the horrible clicking noises of the skeletons grows louder behind them...

Trust me, if you narrate that well and make sure no one dawdles while thinking for too long you'll have your players on the edge of their seats!

Ideally your skill challenges will offer more varied solutions than just "defeat door", but this is something to tailor to your party. And speaking of...

4. Offer "Shortcuts"

Decision making isn't fun if there's only one choice. Split your crypt up into multiple paths, let your members make perception, investigation, survival and other applicable checks to find shortcuts or workarounds for some members but not others.

Also, allow your party to impede the skeleton's progress, maybe with an acid spill on the floor or by caving in a bit of the ceiling. Each time they come up with something clever, don't have the skeletons gain on them that round.

As an example, in one chase I ran there was a narrow alley that our ranger could fit in but our fighter couldn't - the ranger ran ahead and kept sending Message back to the fighter with tips about what was coming. I gave the fighter advantage on her rolls since she had been informed what was coming up. Stuff like this gives each member a chance to shine for their own skill set.

5. Pulling it all together

Once you've mapped out all your decision points and skill challenges you can make yourself a little cheat-map with each space and what will happen in it. Whip up a little token for your party and another for the skeletons and keep track of when the party does something clever and gains a space, or fails a challenge and the skeletons get closer. This doesn't have to be something your players see, just use it to keep your narrative accurate.

Then it's all up to your narration and how quickly your players can think on their feet! If they play their cards right and are clever about it, they'll never actually have to fight a skeleton and you will have built an amazing encounter!

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I can't believe I found this but I actually have a picture of one of the encounters I built.

The left sheet has my "spaces" mapped out so I can track how far ahead/behind my players each are.

The right sheet has the decision points listed (in this case right-to-left) with notes to me about where shortcuts are and what skill challenges are needed.

My players never saw any of this, it was just info so I knew what was happening and could narrate accordingly.

Answer 2

To answer your first question, page 82 of the DMG has rules for how the number of enemies of a given CR scales the difficulty of the encounter.

To correctly gauge an encounter's difficulty, multiply the total XP of all the monsters in the encounter by the value given in the Encounter Multipliers table. [...] When making this calculation, don't count any monsters whose challenge rating is significantly below the average challenge rating of the other monsters in the group unless you think the weak monsters significantly contribute to the difficulty of the encounter.

The Encounter Multipliers table looks like this:

Encounter Multipliers

1 = x1

2 = x1.5

3-6 = x2

7-10 = x2.5

11-14 = x3

15+ = x4

A regular ol' run-of-the-mill skeleton (CR 1/4) is worth 50xp, so assuming you had 15 skeletons, you'd end up with 15 x 50 x 4 = 3,000xp, which would be a Deadly encounter for a party of four level 4 or lower PC's, Hard for level 5-6 PC's, Medium for level 7-10, and so forth.

Now, there's something to be said for the fact that not all the skeletons would be attacking en masse, since they wouldn't all be animating at the same time. On page 83, there is a note on 'multi-part encounters':

Sometimes an encounter features multiple enemies that the party doesn't face all at once. For example, monsters might come at the party in waves. For such encounters, treat each discrete part or wave as a separate encounter for the purpose of determining its difficulty.

This is where it gets tricky, because this rule functions on the assumption that each wave is completely separate from the previous one (i.e. no skeletons are left over from the first wave when the second one attacks). Since this likely wouldn't be the case, the difficulty would dynamically change as soon as new skeletons arrived.

One solution might be to limit the total number of skeletons appearing at any given time based on what would be a Medium difficulty encounter.

• For four PC's of 2nd level, five skeletons would be a Medium difficulty encounter (50 x 5 x 2 = 500xp, Medium would be 400-600). Assuming the party is killing skeletons as they move, maybe have one skeleton animate per turn (to replace the skeleton killed the previous turn).
• To answer your second question, if you were to use CR 0 monsters instead to represent the skeletons (commoners for example), you could manage around 14 or so 'skeletons' (10 x 14 x 3 = 420xp) for a Medium encounter. Assuming the party is killing skeletons as they move, maybe have three skeletons animate per turn (to replace the skeletons killed the previous turn).

This is just based on the math in the book; I personally would drop the total number a bit just to be on the safe side without overwhelming the party (3-4 real skeletons or 10-12 commoner-skeletons).

Answer 3

I think that your kind of approaching it the wrong way. Think of more like a zombie movie theme.

You have your goal. Get our of the dungeon NOW.

As DM you can adjust the difficulty of each encounter, add more encounters, etc. as you need to to keep the tension up. Think of the fights more as traps and less as XP gaining situations.

DM > The boss dies and drops the ruby of significance. But all of the sudden you hear the sound of bones on stone. You look around and see several skeletons starting to animate. They start moving in your direction.

PC1 > I draw my sword.

DM > As you sword shines in the light of the torch you notice more moaning and bone sounds from down the hallway. One thing is for sure. You can't afford to stay here.

PC2 > Grab the ruby of significance

DM > Having recovered the gem you too become aware of even more bone on stone sounds. It's coming from every where.

PC3 > Locate the exit

DM > You can see, threw the growing number of skeletons the chambers exit. As you anr your party start to move towards it, a skeleton grabs you by the sholder.

PC4 > Hack off the skeleton's arm.

DM > You manage to cleave the arm off the skeleton with little effort, you continue to press forward, towards the exit. After a few steps, you notice three skeletons blocking your path, you don't have time for this.

PC1 > ram into the group of three skeletons.

DM > The skeletons in front of you fall to your charge, but the ones behind you are closing in.

And so on.

The thing here is that the urgency exists, but the "fights" are not "I swing my sword" fights but ways to avoid traps. Allow creative solutions like novas and what not, but don't tall into the "combat". A move either works and moves the party forward or doesn't and sets them back. The entire "escape" should award XP not each fight. Keep the tension high in your story telling, forget about the mechanics of the fight.

Answer 4

It depends on the exact situation and party balance

Let me tell you a story.

My adventuring party of a Wizard, Rogue, and Druid (all level 4) recently faced off against 8 Skeletons, a Skeleton Minotaur, and 2 evil Clowns (it's a bit of an interesting campaign). I'm not sure about the Challenge Rating for the clowns, so let's assume CR 1/8.

That's a total XP count of 900, with an encounter multiplier of x3, for a total of 2,700.

That would be a horribly deadly encounter for three heroes if we faced it all at once. But there were two details which turned this encounter from deadly to simple. First off, the monsters did not start out in the area. They arrived over the course of several turns. They weren't quite spaced out for the full "wave" separation that J. Fox mentioned, but it definitely reduced the difficulty.

Secondly, my Druid lay Spike Growth across the entrance that the enemies were arriving from. 6 of the skeletons never arrived, and everything that made it through was badly weakened.

Different teams will do well against different types of encounters

A rogue that specializes in massive damage against single foes will do poorly against a wave of fifty 1 hp skeletons. On the other hand, a Druid sitting in the middle of a Spike Growth circle will laugh and laugh as thousands of bones pile up around the edges. (At least until they bring out their shortbows). The terrain matters a great deal. If your team is ranged and sitting atop poles they will destroy any number of unranged foes, and a single sentinel can hold a choke point against an army.

Difficulty calculators are useful tools, but they are never more than approximations. Use your judgement to see how this particular team will handle different types of encounters and adjust accordingly. And expect them to screw up the balance anyways, because that's what players are best at.