Dungeon Mastering 101: Building an Encounter

Your group’s next game is coming up in a few days and you’re sitting down to create an encounter. Time is short this week, so you think about the shortcuts you’ve heard about for encounter design. Supposedly, one creature with a challenge rating equal to the average level of your party of four PCs will provide a suitable challenge. They’re level 9, so you pick a CR 9 creature: a young blue dragon. Dragon fights are always epic, right? It should make a great fight!

Only, when it comes time for the fight, the dragon fails its save against the wizard’s hold monster spell, dropping to the ground paralyzed. With advantage on all of their attacks, the fighter and barbarian can easily defeat that beefy 18 Armour Class you thought would keep the dragon relatively safe, and soon its 152 hit points are gone and the party only needs to spend a short rest to recover the damage they took from the dragon’s opening breath weapon.

Well, that didn’t work out. This was supposed to be an epic fight! What went wrong?

In this article, we’ll look at the art of designing encounters, including why some of the “conventional wisdom” about this practice is wrong, and offer some tips to make your encounters challenging for your players.

Challenge Thresholds and the Adventuring Day

Pages 81–85 of the Dungeon Master’s Guide are devoted to a handy section called “Creating Encounters”. Of particular salience to this article is the heading “Creating a Combat Encounter”, which demonstrates how to use the XP value of a target or targets to determine how difficult a fight will be for your players and to build an encounter according to the desired challenge. It also offers a way to adjust for the added challenge of facing multiple monsters and also gives a target for the total cumulative experience a character of a given level should face in a single day in order to feel challenged (“The Adventuring Day”).

Using these tables, we can see that a party of four 9th-level characters would have an easy time with an encounter worth 2,200 XP, would face one or two close calls with an encounter worth 4,400 XP, have a slim chance that one or more characters might die in an encounter worth 6,400 XP, and face serious risks to their survival in an encounter worth 9,600 XP. To be challenged over the course of a day, they should face around 30,000 XP in various encounters, having two short rests along the way.

The following table shows the difficulty rating of an encounter that pits four characters of a certain level against a single creature with a challenge rating equal to their own level, as well as of a challenge rating one level above and below their own, and the percentage of the adventuring day XP such an encounter represents.

Level  CR -1 CR Equivalent CR +1
1 Easy (8%)* Medium (16%) Deadly (38%)
2 Easy (4%) Medium (19%) Hard (29%)
3 Easy (9%) Medium (15%) Hard (23%)
4 Easy (10%) Medium (16%) Hard (26%)
5 Easy (8%) Easy (13%) Medium (16%)
6 Easy (11%) Easy (14%) Medium (18%)
7 Easy (12%) Easy (15%) Medium (20%)
8 Easy (12%) Medium (16%) Medium (21%)
9 Easy (13%) Medium (17%) Hard (24%)
10 Easy (14%) Medium (16%) Medium (20%)
11 Easy (14%) Medium (17%) Medium (20%)
12 Easy (16%) Medium (18%) Medium (22%)
13 Easy (16%) Medium (19%) Medium (21%)
14 Medium (17%) Medium (19%) Medium (22%)
15 Medium (16%) Medium (18%) Medium (21%)
16 Medium (16%) Medium (19%) Medium (23%)
17 Easy (15%) Medium (18%) Medium (20%)
18 Medium (17%) Medium (19%) Medium (20%)
19 Medium (17%) Medium (18%) Medium (21%)
20 Medium (14%) Medium (16%) Medium (21%)

* CR 1/2

Looking at these numbers, one can see that mathematically, a creature with a challenge rating equal to the a four-player party’s average level should offer a medium encounter at most levels. So what went wrong with the dragon fight? A medium encounter should have “one or two scary moments for the players”, but this dragon didn’t even offer that much of a challenge. What’s up? Well, in this situation, the Dungeon Master did not use the numbers properly.

Beyond The Numbers

As with any tool, experience thresholds are useless if not properly applied. And proper application of these experience thresholds takes into account that the sum of the experience is not nearly half as important as what the experience represents. Balancing encounters is not so easy as “six of one, half a dozen of the other”; it requires some deliberate planning of what you’re using in the encounter.

Below are some considerations that the experience thresholds don’t include:

The Action Economy. This is where most Dungeon Masters get mixed up, and what primarily went wrong in planning the encounter with the dragon. Unlike older dragons, a young blue dragon has no legendary actions, and so it was completely outmatched facing a group of four adventurers, who have four times its action potential. Alone, even a monster with a much higher challenge rating is vulnerable to the predations of a group, much like how a pack of wolves can take down a mighty elk or our own ancestors hunted woolly mammoth to extinction. This is why monsters that are intended to be faced alone (that is, legendary monsters) have legendary actions; they are balanced to be able to act nearly every turn when facing a typical (four-person) group, thus making for a more even action economy. This allows them to keep dishing out damage as they receive it instead of just sitting there absorbing damage. (They also have legendary resistances so that if they fail their save against effects that would negate their threat (such as hold monster), they can choose to succeed instead.)

The Adventuring Day. If you continue reading past the experience thresholds in the Dungeon Master’s Guide, you will shortly come to a section called “The Adventuring Day”. This is an extremely important section that too many people ignore, and it can easily thwart any attempt to create a suitable challenge. In short, the more encounters that the adventurers face in a day, the weaker they need to be. Throwing five hard challenges at your players in a single day will quickly burn their resources and leave them overwhelmed, meanwhile throwing three medium challenges at your players in a day will barely phase them. If you want to keep your encounters at the right balance, you need to aim for this target.

Damage. It may be tempting to throw a single high-level monster at a party because its XP value seems to make it a good challenge. For example, a single frost giant against a 5th-level party of five would come in just over the deadly threshold, which would seem to make for a dramatic encounter that the party could survive with “good tactics and quick thinking”. And it would be, until the frost giant dispatches the 47-hit-point fighter (the party tank) in a single round and leaves the rest of the group scrambling to outrun the giant’s 40-foot movement while somehow chipping away at its 138 hit points. Rather than be a significant challenge, it’s well beyond the party’s strength.

Special Features. A melee party without spellcasters is going to be severely disadvantaged fighting against things like swarms (which resist bludgeoning, piercing, and slashing damage, even from magical weapons, and want to be up close with characters), while a group that relies on its spellcasters to deal heavy damage will be severely disadvantaged if they face an enemy like a rakshasa before they have access to 7th-level spells. When designing an encounter, you have to consider what the party’s strengths and weaknesses are in order to ensure that you challenge (but don’t foil) their abilities.

Combat Environments

The environment of a fight also has a significant impact on how it plays out, possibly even more so than the experience threshold of the encounter. How often have you found an encounter drags on as each side simply throws dice at the other, lacking any other available strategies? How many times can your fight be described as “the characters fighting a monster in a big square room”? If you struggle with making engaging fights, part of the problem might be that the environments are just not all that exciting.

There are three kinds of features that you can add to make a combat encounter more dynamic and engaging: those that benefit the players, those that benefit the enemies, and those that can benefit either side. The best way to build an epic encounter is to use a mix of all three.

Let’s look at an example combat environment. A 3rd-level party of four (a cleric, a fighter, a rogue, and a wizard) have interrupted a cult ritual being performed in an abandoned mine. The DM has planned for this and included enough enemies to make this a hard encounter with some extra complexity to increase the difficulty to deadly. However, because the enemies he’s selected (mostly acolytes and cultists) have only 9 hit points, making them easy targets to be dispatched by the greatsword-wielding fighter with Strength 18 (and therefore an average damage of 10 slashing per hit) or the wizard’s favoured spell (flaming sphere), the DM has spread them out, given them hand crossbows in addition to their scimitars, and placed several on upper levels out of easy reach, as well as devised an ongoing ritual that the wizard must disrupt, thus occupying some of his attention.

Ritual. While the wizard could probably do a lot to even the playing field with scorching ray or another spell, stopping the ritual becomes an imminent concern that the wizard is uniquely equipped to handle because of his spellcasting abilities and proficiency in Arcana. As an action, the wizard can attempt to disrupt the ritual by expending a spell slot and channelling the power into twisting the magic, giving the cultists performing the ritual disadvantage on their checks. This also reveals that there are three crystal focuses the cultists are using to power the magic, and that they require at least two to complete the ritual.

Crystals. The rogue is well equipped to deal with these, since thieves’ tools can be used to remove the protective runes that would harm someone who attempts to remove the crystals from where they are ensconced. It takes an action to attempt a DC 15 Dexterity (Thieves’ Tools) check to defeat the defence.

Ropes. Forcing the fighter to climb is boring, and so the DM has included a way for the fighter to get up with more panache: several ropes run through pulley systems to support large wooden crates that have been hoisted to the upper walkway for unloading. By cutting a rope beneath where he grasps it, the fighter will cause the crate to fall, lifting him up in its place if he succeeds a DC 10 Strength (Athletics) check to maintain his grip and make the landing.

The addition of these features turns this from a rather boring, straightforward encounter of “kill them before they kill us” to a multilayered, complex encounter that challenges the players to think about what they are trying to accomplish and how they can utilize their abilities to that effect. This is the kind of encounter your players will be happy to have at the end of a story arc and will stand out as exceptional.

Challenge vs. Spite

All too often, we see people complaining that their players find every encounter a cakewalk, to which there are 98,134,482 or more replies encouraging the Dungeon Master to do things like throw some arbitrary antimagic fields into play or make something immune to non-silvered weapons. This is not challenging the party, this is being spiteful. It is BAD PRACTICE and will only make your players frustrated.

The answer to every “overpowered” spell is not to put an enemy with counterspell in every fight. The answer to rogues coating their weapons with poison is not to give everything poison immunity. Denying resources is something that should be done sparingly, otherwise the players will lose interest in gaining new powers since they will simply be countered.

If you find that every encounter is playing out the same way, chances are you are not designing the encounters to be robust enough. While there are a few spells that never should have been printed the way they are (we revised them in volume 6 of Arcane Emporium), there are many others that we have seen become the subject of complaints that were easily resolved by simple adjustments to encounter design.

Conclusion

Creating a great encounter is rarely as easy as matching up some numbers. While the experience thresholds in the Dungeon Master’s Guide can offer sound guidance, they don’t tell the full picture of how an encounter will play out. The “rule of thumb” that a creature with a CR equal to the party’s average level is specious beyond defence; unless the characters have been pushed for their full adventuring day, even a legendary creature with a CR equal to the party’s level can scarcely offer a significant threat.

It is only when you use these thresholds in combination with proper selection of enemies that you are certain to be able to offer your players an exciting and memorable fight. The right enemy is one who adds something new to the fight, something that forces the players to change their strategy; it varies for every encounter, so you need to think about each fight from many different angles. Just don’t lose sight of your PC’s strengths and weaknesses.

If you still feel like your encounter needs help, think about the setting. A fight in crisscrossing treetop pathways or atop a waterfall will add new dimensions to combat that will provide a welcome relief to the ubiquitous 5-foot corridor. A fight that features a race to the summit of a pyramid or a ritual that must be disrupted poses unexpected challenges to the players to figure out where they want to direct their efforts. Look at those stats on a character sheet that aren’t “Attacks” and “Spells” and you can find all sorts of ideas waiting to be used.

And don’t forget: if it’s fun, it’s not wrong.

1 thought on “Dungeon Mastering 101: Building an Encounter”

  1. Beautifully written article.

    I would have liked to have seen a section about how the players and monsters are controlled though. A weak party can destroy a frost giant if they defeat it in some manner other than direct confrontation. A powerful party can be flattened by kobolds if the kobolds use tactics that prevent recourse. The CR system and XP thresholds only account for head-on conflicts, they can’t possibly account for actual strategic skill.

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