Lately I've been playing a lot of D&D online, and the games often only last for the duration of a single module. Groups come together to play through a 20 or 30 page adventure, the game lasts for a few sessions, and then ends. (My current online gaming group, admittedly, was only supposed to game together for "a month or so" and we recently celebrated our 2 year anniversary...) The problem is that I often find myself cobbling together a campaign from unrelated modules, and those modules don't always align with current character levels. To solve that mismatch, you have to scale the adventure module you're playing.
I've scaled up my fair share of modules, and in this post I discuss a few tips I've picked up along the way.
First, some important terms.
Average Player Lever (APL) is the sum total of player character levels, divided by the number of players. If you have 4 players at level 10, then the APL of the party is 10 because (10×4)÷4=10. If you have 4 players, and two of them are level 5 and the other two are level 10, then the APL is 8 because (5+5+10+10)÷4=7.5.
Tiers are the arbitrary way that many publishers group levels together:
When you're running a module designed for player characters of a lower APL than your players, incidental threats in the module probably don't seem very threatening. A trap dealing 2d8 damage can kill a Tier 1 player character, but for Tier 3 or 4 character with 100+ HP, it amounts to a stubbed toe. Buff that with a few healing spells, and it's more like a paper cut, only less annoying.
The Dungeon Master's Guide (DMG) anticipates this, and on page 121 there's a table to adjust how deadly a trap is. When the proscribed damage of a trap seems wrong, use this table to set its severity based on the APL.
Sometimes, I just multiply the number of damage die on a trap by the Tier of the player. For instance, say a trap is written as 2d8. I multiply 2 by either 1, 2, 3, or 4.
Caution: Just making numbers bigger feels like video game logic. When scaling damage for traps and similar effects, have a good reason for the damage being higher than written. Your players probably won't know what's written either way, but any player who remembers their first spike pit trap will know they usually do 2d10, so why at Tier 4 did the same trap deal 8d10? They must have fallen farther, or there had better be an ooze or pool of acid lingering around the spkes, or some flamethrowers that trigger upon impact, or something.
When I'm running an adventure writter for Tier 1 play for Tier 2 players, I usually just swap out a low challenge rating (CR) monster for something with a higher CR. Instead of using the CR 3 monster written into the module for 6th level players, I use a similar monster with CR 6. Between all the bestiaries on my bookshelf, I have plenty to choose from.
However, after Tier 2 you're likely to find that just swapping monsters according to CR isn't as effective as it is at low-level play. Whether you're playing Pathfinder or 5e, at some point the fact that your players outnumber the single monster you're throwing at them starts to really matter. You can throw a CR 18 or CR 20 monster at even 15th level characters, and they'll get rid of it in a single round, because all attacks and all spells are focused on that one monster. Even if the monster has three attacks and a legendary action, that's still just one attack for each player character (and statistically, probably only a few will be successful).
Within a single round, that's 4 player attacks against 1 monster compared to 1 monster attack against 1 player character. 4-to-1 is very likely to defeat 1-to-1.
The trick is to force the players to distribute their attacks, and the way you do that is, quite simply, by adding additional targets. There are two strategies for this.
First, you can have one big monster and lots of little monsters. Count on the little monsters dying quick and early. Their only purpose is to draw fire away from the big monster, and to deal whatever damage they can in the mean time. Clever players may know to ignore very small monsters, so adjust your definition of what a distraction is to a high-level character.
The other method is to have more than one big monster. That doesn't mean you can throw two CR 15 monsters at a 15th level party, but it does mean you can use two CR 7 monsters, or maybe a CR 7 and a CR 8.
On page 90 of Xanathar's Guide to Everything, there are what I think are some of the most confusing tables ever created to help you budget your monsters with relative precision. It takes me about 10 minutes every time I look at the tables to remember what they're trying to describe, so mostly as a favour to my future self, here's what I hope is a lucid explanation.
On the Y-axis of the tables is the level of exactly 1 character in your party.
On the X-axis is the challenge rating of exactly 1 monster.
Imagine 1 character and 1 monster on a set of those old-timey classical-looking scales. The scale is balanced as long as the character side weighs as much as the monster side. The table describes this scale. The values in the table are never unbalanced, and in fact they tell you how many more characters or monsters you need to add to the scale to maintain perfect balance. The character is always on the left, and the monsters are always on the right.
When you see 1:1 in the table, it means 1 character of the level shown on the Y-axis is exactly equal to 1 monster of the CR shown on the X-axis. The table assumes there are 4 players in a party.
A 1:1 balance for a 1st level character is a single CR ¼ monster. To maintain balance and use a CR ⅛ monster instead, you need a 1:2 ratio (1 character to 2 monsters).
A 1:1 balance for a 6th level character is a single CR 2 monster. To maintain balance and use a CR ½ monster instead, you need a 1:5 ratio (1 character to 5 monsters).
I guess another way to think of the table values is that it shows the "value" of each player character in a party. If you have 3 players, and they're all 6th level, then you know exactly how many monsters, at each different possible CR, each player is worth. Even better, if you have 3 players all at different levels, then you still know exactly how many monsters each player is worth.
I think one of the most common ways of calculating monster encounters is to quickly generalize the CR level split, and then adjust during the game to account for "budget" errors. In other words, say your average party level is 10, so you know you need a CR 10 threat. Your players are pretty powerful, though, and the combat lately hasn't felt very challenging, so you decide you want to make things a little more dynamic. Instead of a CR 10 monster, you pit your players against a CR 2, CR 3, and CR 5 creature. That's a total of CR 10, with the added benefit of there now being more than one target for players. Now, when you start playing the encounter, you might find that the added targets pushed the encounter over the CR 10 edge a little.
Maybe you should have gone with two CR 5 monsters instead, or a CR 10 with two CR ½ monsters to annoyingly bite at the heels of the player characters. If that's the case, you can adjust as you play, in whatever way you're comfortable. Maybe you ignore a special attack feature that the CR 5 creature has, or maybe you use average damage instead of rolling for damage, or maybe you play the monsters a little less tactically than you normally would.
Or you could just admit you were over-zealous in your encounter design, and kill a player character or two, and resolve to do better next time. It's just a game, and while resurrection is expensive, new character sheets are just a photocopier and 6 dice rolls away.
Players have to adjust their game play, too. They have to change their prepared spells, take new feats, they have to acquire better gear and armour, they have to learn to think outside the box to anticipate new challenges.
Adjusting an adventure as a Dungeon Master isn't a chore. It's the equivalent of maintaining your character sheet, except the character sheet doesn't describe your character, it describes the world. With experience, scaling an adventure becomes second nature, but I don't think it ever becomes predictable, and you don't really want it to be.