Wednesday, January 17, 2018

Damage Reduction in 5th edition

Not exactly
This past weekend, I ran Broodmother Skyfortress, which is an utterly fantastic module from Jeff Rients.

In it, you fight Giants. I'm not going to talk much about them here, but Jeff recommends you give the Giants a special ability: Damage Reduction instead of Armor Class. This is to give the impression of an "alien" monster, something that doesn't quite work the way the PCs do.

So, how did I decide what that DR was for a 5th Edition version of the monster?

Well, it all comes back to one of my favorite charts in the Dungeon Master's Guide: Monster Statistics by Challenge Rating, page 274. On it, you can find the baseline AC, HP, attack bonus, damage, and saving throws for monsters at each CR. As I've discussed before, this allows you to make and modify monsters with some idea of what the end result will be.

So, essentially, we are just adding a column to this table for Damage Reduction. This turned out to be a lot more complicated than I expected.

Thanks to Bounded Accuracy in 5th edition, you have an average of 55-65% chance to hit at every level. Let's round that to 60%. This means that for every 10 damage you could dish out, a monster's AC effectively will block 4 of that damage. So for a monster with 50 hit points, we'll need to block about 20 points of damage (40%) with the DR to make it equivalent. Call it a DR Threshold.

The D is for DANK
That 20 points of damage reduction is going to have to be divided among every attack and spell the party makes. With an average of 5 players in a group and 3 rounds in a combat (these are the DMG's assumptions for all Monster abilities), we can assume a given monster will take about 15 turns of combat to defeat.

This will also have to be modified based on how many attacks a particular player can make on a turn. If our DR is set to 4, a fighter with 2 attacks will actually account for 8 points of damage reduction in our final calculation.

Nearly all martial classes get an extra attack around level 5 or 6. This means we'll have to make an adjustment when the monster hits CR 5 or so.

Now, what about 3rd, or 4th attacks? Those are mostly the purview of the fighter and monk. Other classes tend to focus on beefing up a single attack, via Rage, Divine Smite, Sneak Attack, etc. So beyond this, we won't need a change.

But we still need to account for two things: critical hits and large damage spells. After all, if a PC gets a critical hit, they are basically skipping a turn of DR that would contribute to our total DR. And if a wizard drops a Fireball, that's quite a bit of damage for only one DR.

A critical hit (based on our 15 turns in a combat) will probably happen once in a single fight. With more attacks, they become more likely, but not every character will have more attacks.

A massive damage spell, on the other hand, will likely happen every fight if you have a Wizard or Sorcerer in the party. So we'll need to account for that. However, due to spell slot limitations, we can safely assume it will happen no more than twice in a single fight. And that damage will likely be 2-3x as much as a single attack would be.
Oh no an overpowered spell *cough cough* Fireball *cough*
These factors have the effect of lowering the number of turns we have to achieve the total DR we're looking for. So, although we are assuming there will be 3 rounds of combat, the DR threshold needs to be met in 2 rounds, to account for large sources of damage.

This gives us our formula:
  • (Average Monster Health at a given CR) * 40% = DR Threshold
  • DR Threshold / 10 turns = DR per turn (always round down)
  • At CR 5 or greater, DR per Turn / 1.38 (to account for some classes having an extra attack)
  • At CR 20, DR is capped (as per AC in the DMG)

And by using this formula, we can get our table:
  • CR 0: DR 1
  • CR 1/8: DR 1
  • CR 1/4: DR 2
  • CR 1/2: DR 2
  • CR 1: DR 3
  • CR 2: DR 3
  • CR 3: DR 4
  • CR 4: DR 4
  • CR 5: DR 4
  • CR 6: DR 4
  • CR 7: DR 4
  • CR 8: DR 5
  • CR 9: DR 5
  • CR 10: DR 6
  • CR 11: DR 6
  • CR 12: DR 7
  • CR 13: DR 7
  • CR 14: DR 7
  • CR 15: DR 8
  • CR 16: DR 8
  • CR 17: DR 9
  • CR 18: DR 9
  • CR 19: DR 10
  • CR 20 and higher: DR 10
If a creature's DR is greater than its suggested DR, increase or decrease its defensive CR for every point of difference. This will allow you to create truly impenetrable hides on your monsters.

It's not only useful to the thick-hided, of course. Use this when you're creating walls of solid stone, or clouds of gas that you literally can't miss, or other things that might not take the brunt of your attack but are simple to hit.
Or if you're just super thicc
As I wanted my Broodmother Giants to be around CR 10, I gave them DR 6.

Thanks for reading!

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