The encounter creation guidelines in the Dungeon Master’s Guide and Xanathar’s Guide to Everything are both based on the idea that you know how many PCs are in your group. Then you do a table lookup, do a little math, and – presto! – you have a budget to spend on your encounter, expressed as either an XP amount or a number of creatures of a particular challenge rating.

But what if you’re using a published adventure with a party that’s a different size than the one recommended? For example, what if you’re running Curse of Strahd (“for four to six players characters”) but your group only has three PCs?

Going strictly by the book, you would need to deconstruct the encounters in the book to calculate the original XP budget, determine what difficulty the encounter was designed for (Easy/Medium/Hard/Deadly), calculate the correct XP budget for your PCs, and then rebuild the encounter using the new XP budget. (Which may or may not be possible with the original creature(s) used in the encounter.)

Here’s the tip: It’s a lot easier to adjust the level at which your PCs play the adventure than it is to redesign every single encounter.

• For Tier 1 & Tier 2 characters, increase their level by +1 for each “missing” PC from the party.
• For Tier 3 & Tier 4 characters, increase their level by +2 for each missing PC.

Or vice versa for additional PCs.

To put that another way, if a published adventure’s recommended level is X, then at Tier 1 & 2 use it for PCs who are level X + 1 per fewer PC and Tier 3 & 4 use it for PCs who are level X + 2 per fewer PC.

So if you’re running Dragon Heist, which is recommended for five PCs, with a three-person group, you’d either want to start them out as 3^rd level characters (instead of 1^st level characters) or run a prequel adventure or two to level them up to 3^rd level before using the published Dragon Heist campaign.

BONUS TIP #1

Challenge ratings are not that precise. They’re not designed to be a guarantee (nor can they be). They are a very rough approximation of “on average.”

Some “balanced” encounters will be easy. Some will turn out to be surprisingly difficult.

That’s okay. No game, no adventure, no session is about a single encounter.

The flip-side of this is that you don’t need to worry too much about getting an encounter exactly right. It’ll mostly get washed out in the general noise – the imprecision of the system, encounters being designed over a spread of challenge levels, situational conditions of the battlefield, and so forth.

This is also why Wizards of the Coast can release adventures “for four to six player characters” of a given level. Such adventures are designed for five PCs. They’ll be a little bit harder for four PCs and a little easier for six PCs, but it’ll be just fine.

BONUS TIP #2

In terms of strict math, the rule of thumb described here breaks down for very large groups of 9+ PCs. But there are more significant balancing issues based on action economy that make creating and running encounters for such large groups more of a special snowflake in any case. (Short version: Ten PCs, with all their attacks and all their special abilities, are able to wreak an amount of havoc that is out of linear proportion to a group of four. But, conversely, you can’t just use more powerful creatures, particularly at lower levels, because the monsters can one-shot individual PCs before they go down.)

For groups of 8 PCs, rather than running higher-level adventures, you can get pretty good mileage out of taking an adventure designed for five PCs and just doubling the number of creatures in the encounter.

For groups of 9+, adjust the encounter based on the difference between the PCs’ group size and a group size of eight, and then double the number of creatures. (This breaks down somewhere in the teens, but I would… uh… strongly recommend not running groups that large.)

This can create some weird narrative challenges if the encounter was, for example, with a solo boss or the like. But those are the types of encounters which really don’t work with large groups in any case, so you’ll just need to give them a little more TLC.

BONUS TIP #3

Whatever approach you’re taking to encounters — prebuilt, custom built, or otherwise — remember that you can always dial it in over time: If the encounters you’re building are too hard, trim the XP budgets in the future (no matter what the by-the-book math says). If you’re running a published adventure and the PCs are steamrolling the opposition, hold back on leveling them up. Or, vice versa, level them up faster if they’re struggling.

As you’re getting a feel for things, keep in mind that you have to miss by A LOT and for a very long time for “too easy” to not be fun.

You only have to miss once for “too hard” to be a campaign-ending TPK.

So erring on the side of easy is recommended. You can dial it up from there.

Random GM Tips: Running Combat Revisiting Encounter Design Random GM Tip – Cypher Bad Guys