ThinkDM

AoE Spells vs. Hordes

Added 2020-01-15 02:58:20 +0000 UTC

The best way to make a spellcaster feel powerful is to throw hordes of enemies that the caster can dispatch with a single blast. The problem is that actually running this math at the table can be an absolute chore. The time gap between rolling a ton of damage dice and actually narrating what happens leaves the experience a little flat.

In order to fix this, we need a faster way to adjudicate area effect damage against hordes of monsters. Taking a cue from Mob Attacks in the Dungeon Master's Guide (p. 250), we can reverse-engineer the odds of an attack success to determine how many mobs it would take to hit. For mob saving throws, we just turn that formula inside out.

Spell Efficacy vs. Hordes

Here are the odds of a getting the full effect when casting a spell that triggers a saving throw. This represents a single creature's probability of failing their save, as well as the percentage of targets in a group that will fail their save.

Note: The 19 is flagged in red, because that's your maximum spellcasting save DC without items. A Wand of the War Mage can boost that up to 22.

This is great for figuring out how many creatures in a horde will be affected by the spell effect. But, it doesn't tell the complete picture for damage.

For damage spells, making a successful save results in taking half damage. When we half the damage for the targets that saved and add it to the full damage for the targets that failed the save, we get the total percentage of the max damage we would normally deal.

Next, we need to put this chart to use. It's great to know that we can reduce our rolled damage by a certain percentage and skip rolling the saves, but that's really mathy and doesn't necessarily make things easier to play at the table.

In order to do that, we need to figure out two parts of the equation:

What monsters tend to run in hordes?
What spells are we likely to target them with?

Monsters

I've pooled a collection of monsters that are likely to become targets of AoE spell effects, since they run in hordes. These are also popular options for filling out minions among a more interesting high-level encounter.

Knowing what monsters to look at only gives us part of the picture. We need to know what stats are relevant. Obviously, Hit Points (HP) are going to come into play. Beyond that, we need to look to the spells to know what kind of saves will be targeted.

Area of Effect Spells

The majority of area effect spells in 5th Edition target Dexterity or Constitution saving throws. There are a few exceptions (which may be expanded if the psionics Unearthed Arcana is any indication), but for now it's the best use of our time to focus on the Dexterity and Constitution saves for the mobs we'll be targeting.

Constitution Saving Throws

Dexterity Saving Throws

Other Saving Throws

Putting It Together

I compiled a chart with the HP and saves of the selected monsters. Based on the save DC of the spellcaster and the saving throw bonus of the target, we can look back to the chart that tells us the effective percentage of damage we deal. If we divide the monster's HP by the percentage of damage the spell does, we get a modified HP total that takes into account the saving throw bonus.

Since each monster in range of an AoE spell would take full damage, a damage roll that does the full amount of the monster's adjusted HP should take down all monsters in range.

If we don't roll that high and need to calculate how many of the targets we take down, then we can calculate a good rule of thumb by dividing the total mob HP by the number of targets. That's where you get the "per target" number in the chart above.