It has come to my attention that my judgment around what constitutes a challenging series of encounters in Fifth Edition Dungeons & Dragons still requires refinement. It’s easy to frame the question a bit naively as such: “Given an average party level of X, and a monster challenge rating (CR) of Y, how many monsters can the PCs take on?” This is a good question to start with, but designing encounters needs to be about more than whether the PCs can theoretically survive that encounter, and the CRs of the monsters end up being only one piece of the puzzle.
Degree of Challenge
The first extension of this question is how challenging you want to make it. There’s a large spectrum outside of the simple question, “Do the PCs win?” The PCs might survive without taking any damage at all, but if the wizard cast fireball once or twice to open the fight, then that’s a significant resource that has just been expended, possibly just as crucial as the hit points that would have been lost if the fight had gone on longer.
This is particularly important when you consider the fight as just one entry in a series of encounters. A fireball cast by the wizard in the first encounter of the day is a fireball he cannot cast in the last encounter of the day. If you, like many others, like to have the last fight feel climactic by way of also being the most challenging, it’s worth considering that it will be more challenging by virtue of the easy fights along the way.
My session last night was a good example of this. I made up a dungeon pretty much as we went along because one of our players was absent, and we wanted him there for what was coming next in the adventure they’ve been on. This meant I didn’t have a whole lot of time to think about the encounters; encounter designs were made mostly on gut. I ended up with two gibbering mouthers (each CR 2) as the final fight of the dungeon. This wouldn’t have been a terrible challenge, except that by the time they got to the mouthers, the party of three level 4 characters, while in good health, had used up a great deal of their resources, most notably their spells. They beat the mouthers and completed the dungeon, but only just barely.
An important takeaway from that encounter is that I could have swapped the gibbering mouthers with the room full of cultists just prior, and the escalation of difficulty would have felt approximately the same; the sorcerer would have burned through the mouthers as quickly as he burned through the cultists, then the mob of cultists and their caster buddies would have been much more menacing. There was nothing that actually made the gibbering mouthers inherently more difficult beside the fact that the PCs had to fight to get through to them already. This of course begs the question again of how best to handle it when the party decides to take a long rest in order to recover, and what cost should be paid for that. I won’t even try to answer that here.
The Feel of the Challenge
While the challenge level of the mouthers (given their placement in the dungeon) ended up working out appropriately, I didn’t like so much the feel of the encounter. Put another way, I’m not sure I liked the type of challenges they presented. The mouthers have very low AC and comparably low attack bonuses—so they’re easy to hit and rarely hit themselves under normal circumstances—but they have three abilities that are equal parts menacing and irritating. The first two cause the players close enough to the mouthers to make saving throws at the start of each turn, potentially limiting or entirely eliminating their ability to act for that turn if they fail. The third ability can blind multiple players, once again limiting player actions. I personally find these sorts of abilities very irritating as a player if they repeatedly hinder me, so I want them to be rare. But if those abilities are too rare (or too easily resisted), they might never show themselves, and the mouther is instead just a (literal) blob of experience points with really low AC.
I do have to give the mouthers kudos for having mechanics more interesting than numbers. Many monsters are only challenging because they have high AC or lots of hit points, or because they dish out a lot of damage, so a monster that can blind you, immobilize you, or cause you to act randomly for a turn at a time is a fun change of pace. I just think they might not show up again for a little while.
Meaningful Encounters
Sometimes a simple encounter presenting no real threat or challenge to the party can still be meaningful. The first room in last night’s dungeon had two ghouls hiding in murky pools of water. The paladin, being cautious with his approach but no longer cautious with expenditure of resources, located the ghouls with his divine sense class ability. They managed to destroy both ghouls before the undead beasties even set foot on dry ground.
Those ghouls probably wouldn’t have done much to the players even if the players hadn’t gotten the drop on them, but they both set the tone for the dungeon (and made any water in the rest of the dungeon instantly “icky,” if not dangerous, since there might be more undead) and allowed the players to feel the satisfaction of playing smart. Mechanically, they were practically inconsequential, but they felt, to me, like a solid opener for the adventure.
Fake It ‘Til You Make It?
Ultimately, my encounters’ difficulty levels end up panning out right about where I want them to, but only because I change my plans or fudge a rule or a roll occasionally (which I do less these days). Maybe I drop an extra healing potion or change the number of monsters in a fight depending on how the PCs are doing so far, because my projections didn’t hold up. I’ll grant that even under the best of circumstances, it’s impossible to know how a fight will go—sometimes your gibbering mouther, let’s say, gets two critical hits and nearly kills the paladin—but I don’t think I’m at the point yet where I can make any kind of reasonable approximation for the pressure or danger players will feel as they progress through a dungeon. So I make it up at the table.
I’d like to claim to be the deist DM, needing only to follow my world’s internally consistent logic, but I may never become that. Maybe I’m just not that kind of DM.
No Free Parking Money 
This also begs the question of how you measure a challenge ex post. A scenario might be plenty challenging, but a good strategy leave the party unscathed while an easy encounter bruises those who are ill prepared. Do you think of a challenge level as being prepared solely with respect to the character’s or do you take into account the skill that the players bring as well?
Great post, Gibbsy. I’ll reply in kind:
Degree of Challenge
I like your realization that easy early encounters likely make the final encounter more difficult due to resource expenditure. The answered question that you yourself posed, though, is how to deal with “breaks” in dungeon exploration.
I’ll tackle that here, briefly:
First, let’s acknowledge that higher-level parties are typically more capable of taking a break than a lower-level party is.
In general, either a dungeon/adventure needs to be designed with breaks in mind, or, if not, there needs to be a penalty for taking a break. Otherwise, it breaks the difficulty progression of the whole dungeon/adventure (per your post, that’s not very good).
In a pre-published module that I’ve run a few times, the authors designed the dungeon with these concepts in mind. There are about 20 orcs in the dungeon, and about a dozen more that are raiding the countryside. If the PCs kill some orcs in the dungeon, and then leave to rest, the raiding orcs come back to replace the dead orcs. Then, instead of the rest being a “re-charge” it becomes a “re-do.”
Or, perhaps the penalty for taking a break is that the villain leaves/escapes to another hide-out. Then, instead of the PCs doing half a dungeon/adventure followed by the second half of the same dungeon/adventure, they end up doing half of the first dungeon/adventure, and then all of the second dungeon/adventure.
Part of it may just be “training” your players to consider not only the rewards of a break, but the risk of doing so as well. There needs to be opportunity costs involved.
The Feel of the Challenge
As a PC, I likely have a unique perspective, which is that I enjoy hard fights where we almost lose and shit goes sideways. This includes characters being rendered ineffective.
It seems intuitive that, generally, players don’t enjoy it when their characters are rendered useless. I get that.
Meaningful Encounters
That opening encounter seems very useful, as you observe. Setting the mood and rewarding characters for smart decision-making can be hard to do. Even if you did it accidentally (which I suspect is the case), it’s a good outcome. Enemies can make poor plans and poor decisions too, and, honestly, a world in which every enemy doesn’t seem terribly effective or well-prepared likely feels more real.
I’m often jolted out of the immersion of the game, however, when I notice that my character is in an encounter that is far too easy. It can be annoying in the moment to be in a fight that does not at all feel threatening. Unless the encounter adds value in another way (see discussion on mood above; notable value for plot-development here), the overly-easy encounter becomes just a distraction.
An exception from this general rule is that encounters with novel scenery/terrain/geography/geometry are almost always interesting, regardless of the difficulty (e.g. the PCs are river-rafting, dodging rocks while exchanging fire with otherwise-non-challenging enemies along the banks).
Fake it ‘Til You Make It?
I think the better solution is adding or subtracting enemies (or disposable treasure), compared to fudging dice rolls. If you fudge a roll you probably end up feeling like you cheated yourself a bit, whereas fudging the number of opponents–to me at least–feels less like cheating, and more like “difficulty scaling.”
I guess the difference is that once you’ve added or subtracted enemies, you can still feel logically consistent with yourself by “letting the dice fall as they may” (having already fudged the input). Really it feels better to change the input than the output.