New to Sly Flourish? Start Here!
by Mike on 28 December 2020
Note: This article has been updated since the original written in February 2015.
The following tools are intended to make it easier to improvise situations in your D&D games. These numbers are designed to be simple and straight forward enough to keep in your head. You can, of course, write them out on a 3x5 card or sticky note and paste it in the inside of your DM screen as well. These numbers help you create challenges, traps, encounters, environmental effects, and horde battles without needing to look anything up in a complicated chart.
Most of these numbers are based on a challenge rating for the sitiuation. This challenge rating is roughly equivalant to the average character level of a group of adventures between 1 (1st level characters) and 20 (20th level characters). This challenge rating is based on the situation, however, not the actual level of the characters in the game. The world does not conform to the level of the characters.
For more tools like these, check out the Lazy DM's Workbook.
Here's a summary of D&D numbers you can keep in your head:
When a situation comes up requiring a difficulty check, choose a number between 10 (easy) and 20 (hard) as the target. The harder the challenge, the higher the number. A 10 is considered relatively easy yet still challenging enough to warrant a roll. A 20 is considered nearly impossible for most common folk.
This number also works for an improvised armor class and saving throw DCs if needed. If you happen to improvise a trap or an effect of some sort, or the characters start attacking a stone statue, you can use this range to set the AC of the statue or the DC of the trap's saving throw.
Example: The Icebolt Trap
Say you've decided a particular room has an icebolt trap in it. How tough was the wizard who planted the trap? Was he an apprentice or an archmage? Choose a number between 10 and 20 to determine the difficulty of finding and disarming the trap. For this example, let's say this icebolt trap has a DC of 14 to detect and disarm.
Note, we're not setting the trap based on the level of the characters. The world is a dynamic place and the characters are just living there. The world does not change it's DCs based on the characters who face it.
If we ever need to improvise an attack score, choose a number between +3 (not particularly accurate) and +12 (very accurate). Anything lower is going to be unlikely to hit and not worth rolling. There are some situations where the attack is lower or higher than this but this range is likely for most situations. When you have an improvised attack, choose a bonus based on the accuracy of the attack.
Example: The Icebolt's Attack
Going back to our example from before, let's look again our icebolt trap. If a character fails to detect it or disarm it, it fires an icebolt at the one who triggered the trap with a +6 to attack.
If you need to inflict some improvised damage, 5 (1d10) damage per challenge level is a good rule of thumb. It's roughly the challenge faced by four characters so a challenge 6 is the equivalent of four level 6 characters. If this damage would affect more than one creature, reduce it to 3 (1d6) per challenge level. As mentioned before, this challenge rating isn't necessarily based on the level of the PCs but instead the level of the challenge they face.
Note, for the examples below I'm using the average of a die to determine the static damage, rounded down. Thus, 5 is the average of 1d10 but 11 is the average for 2d10 (5.5 x 2).
Example: The Icebolt's Damage
Returning to our icebolt trap example, we'll have to decide how dangerous this icebolt is and choose 6 damage per challenge level. Assuming the goblin wizard was a challenge rating of 3, the ice bolt inflicts 16 (3d10) cold damage. If this ice bolt had been placed by the lich Xathron, a challenge 16 monster, the bolt might inflict 90 (16d10) cold damage instead. The challenge rating of the villain setting up the trap gives you the idea how much damage to dish out.
If you need to improvise hit points for an object, use 20 hit points per challenge level. This doesn't match up perfectly to the hit points of monsters in the Monster Manual or the Monster Statistics by Challenge Rating chart on page 274 of the Dungeon Master's Guide, but it's close enough.
Example: Xathron's Icy Automaton
Let's say the PCs have invaded the lich Xathron's treasure vault and inside is Xathron's Icy Automaton. This isn't Xathron's best guardian, but it's pretty solid. We'll consider it a level 5 challenge.
The PCs fail to notice the Automaton's danger (failed on a DC 15 perception) and it begins to fire icebolts at random PCs (two attacks, +7 to attack, 15 damage). The PCs can't seem to get it disarmed (failed on three potential DC 15 Arcana or Athletics checks) and now they want to bash it down (AC 15, 100 hps). After inflicting 100 damage to it, the automaton falls apart.
Looking at these number ranges, you may be tempted to use them to build a monster. Instead, consider reskinning existing monsters from the Monster Manual rather than building a monster from scratch with these numbers. While you might be able to build a reasonable monster with these scores, the asymmetrical nature of the stats in the Monster Manual makes creatures much more fun to fight than a static box of perfectly aligned scores.
When building combat encounters, you can skip the complicated math outlined in the Dungeon Master's Guide and instead use this simple encounter building benchmark:
First, build encounters based on what makes sense for the story and the situation. Let the story drive the number and types of monsters.
Then, if needed, check to see if the encounter may be deadly. An encounter may be deadly if the sum total of monster challenge ratings is greater than one half the sum total of character levels, or one quarter of character levels if the characters are below 5th level.
This isn't perfect and lots of variable can change up how difficult a battle is but it's a good rough benchmark that, I'd argue, is as good as any of the fancier methods for benchmarking encounter difficultly found in the Dungeon Master's Guide, Xanathar's Guide, Kobold Fight Club, or any other encounter calculator.
Sometimes the stories of our games lead to the characters facing large hordes of monsters. Rolling tons of attacks and saving throws can suck the energy out of what would otherwise be a really exciting fight. The Dungeon Master's Guide includes rules for adjudicating a lot of attacks from a large number of monsters. So does the [Lazy DM's Workbook]Lazy DM's Workbook.
For an easier method requiring no table, we can start with a baseline assumption that when a large force of weaker monsters attacks the characters about one quarter of them hit. Likewise, when a character hits a large number of monsters with a big area-of-effect ability, about one quarter of them make their saving throw.
For example, our party of 8th level characters gets attacked by fifty skeletons. Many of the skeletons slash with swords or fire splintered recurve bows. Split the attacks evenly across the five characters so each character gets attacked ten times. Instead of doing a bunch of comparisons of attacks to AC, we can assume one quarter of them hit. If the character is particularly well armored we round down. If they're wearing lighter armor, we round up. Thus each character takes between 10 and 15 damage when attacked.
Now the cleric casts Turn Undead. We can likewise assume one quarter of the skeletons succeed on their saving throws and three quarters fail and are destroyed as a huge wave of radiant energy blasts them to dust. Now only twelve of the skeletons remain.
We can do a lot of math to figure all of this out but the result is essentially the same after we round it out.
Instead we can just remember a simple rule: when a large number of weaker monsters faces the characters, about one quarter of them succeed on attacks or saving throws..
In summary, here are some numbers to keep in your head:
With those numbers in mind, you have a simple toolbox for running all sorts of challenges for your D&D 5e group.
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