D&D 5e: Adjusting Monster HP

CR	HP	AC
0	1-6	13
1/8	7-15	13
1/4	16-49	13
1/2	50-70	13
1	71-85	13
2	86-100	13
3	101-115	13
4	116-130	14
5	131-145	15
6	146-160	15
7	161-175	15
8	176-190	16
9	191-205	16
10	206-220	17
11	221-235	17
12	236-250	17
13	251-265	18
14	266-280	18
15	281-295	18
16	296-310	18
17	311-325	19
18	326-340	19
19	341-355	19
20	356-400	19

One of the classic ways to put a higher-difficulty monster into a module for lower-level characters is to have it be already injured before they fight it, giving it fewer HP. Meanwhile, GMs often figure that if they just set a monster higher in its HP range (by default, they take the average dice rolls) that’s still fair but might make the fight a little more challenging.

If you haven’t looked into the monster math for 5e (starting on page 273 of the DMG), your intuitions about altering HP might be wrong. If you’ve come from previous editions where the total number of HD was equivalent to the monster’s level, that is no longer the case. The hit dice for a monster in 5e are basically completely arbitrary. The designer was trying to hit a certain HP total, and adjusted the number of HD until the average value of those dice plus Con mod got into the right range.

This means, if you try to adjust the monster’s HP, your intuition might be wrong for how much that lowers or raises the difficulty. Even a monster with extremely low HP might obliterate a lower-level party if it wins initiative (especially if it also has a high AC), since damage scales so high. And maxing out a monster’s HP could functionally double it, which could push it multiple CRs higher in some cases.

The relevant table is on page 274 of the DMG, but the important parts are to the right. Note that most of the published monsters are close to this system, but not exact, as they were likely tweaked in playtesting. But this is the official guideline for how monsters are supposed to be assigned a CR, and it’s at least in the right ballpark most of the time.

Essentially, to figure out a monster’s defensive CR, look up its HP on the table to get the basic CR. Then consult the target AC for that line. If the monster’s AC is different, move the CR up or down by 1 for every 2 points of AC. For example, if the monster has 110 HP and 15 AC, that puts it at CR 3, which has target AC 13. Since it’s 2 AC higher, its defensive CR is effectively 4. Special abilities might also increase this: if it has a lot of resistances/immunities or special defensive actions, it might bump up another couple.

Some monsters are glass cannons (high offense, but low defensive CR) and others are tanks (the opposite), so have their offensive CR and defensive CR averaged. Basically, if you’re looking at a monster before adjusting its HP, and its defensive CR comes out higher or lower than its listed CR, it may have more or less offense to compensate. Just keep that in mind.

To adjust the HP, figure out what CR the new HP total would put it at, then step up or down if the monster’s AC is not in line. Figure out how much that differs from the original defensive CR, and adjust the monster’s CR by half the difference (because you’re averaging the offensive CR).

Examples

The Inert Golem

This is basically what got me thinking about the problem in the first place. The Planescape module Doors to the Unknown‘s first chapter features a battleground where variant iron golems have gotten to 0 HP but not been smashed up, which means that if they take any fire damage, this causes them to heal HP and reanimate, attacking the party. They’re likely to acquire less than 20 HP from the various reduced-damage fireball effects in the area, but how high of a CR is an iron golem with only a few HP?

The base iron golem in the MM has 210 HP, AC 20, and a CR 16. 210 HP is right around CR 10, which expects 17 AC, so the 20 AC bumps that up a couple of CRs to 12. We can probably also assume that the golem immunities and special defensive abilities are good for a couple more points of CR, so about 14.

What if it only had at most a dozen HP from a stray 2d6 mini-fireball? That puts it at CR 1/8, which targets AC 13. The extra 7 points of 20 AC are now four steps up the chart, landing at 2, and the same immunities and such put that two more steps up to CR 4. That’s a 10 point difference, halved to 5, so lowers the CR to 11.

Is 11 too high for something that would go down in a single attack? Maybe? If nobody manages to get through its 20 AC (or its magic resistance, or targets an immunity accidentally) before it acts, it’s going to likely put out about 45 damage when it gets to act. Probably still doesn’t make it better than a Stone Golem (CR 10). It could wreck a low-level party that can’t manage to hit (especially if they don’t have magic or adamantine weapons), but this is an example of why you don’t want the offensive and defensive CR to get too far apart. It would probably make more sense to have a scenario where there’s a full-on fireball barrage that would probably get it to at least another CR threshold or two.

The Two Wolves

Winter Wolves are HP 75, AC 13, CR 3 with no real special defensive abilities other than immunity to cold. That HP total puts them right at CR 1, and the AC is correct for that CR. Which means most of its CR is coming from offense, presumably.

Maybe you want a threat for a first-level party, so try to have an injured wolf. Dropping it to half HP puts it in the CR 1/4 range, which doesn’t alter the AC, so it’s two steps down. That, halved, reduces it to a CR 2. It’s still, offensively, much more powerful: that cold breath could wipe out multiple low-level PCs if they get too close, and its bite might take out one every round. If there are multiple, Pack Tactics can be a big threat as well.

Conversely, you want a really chonky boi that’s been eating well and is at maximum for his (arbitrary) 10d10+20 HD: 120 HP. That puts him up at CR 4, and now his AC is a little low, so maybe a CR 3. That’s two points higher than the base for an averaged +1, and makes him a final CR 4.

5e Background: Sole Survivor

Whatever you were before, your history pales before what you were made to be: the sole survivor of a horrifying event. It might have been a single night of terror, you and your friends stalked through the darkness trying to survive until dawn. It might have been years of slowly-dwindling resistance, trapped in an occupied city. Your tormentors may have been inhuman antagonists, impossible to communicate with or placate, or they may have been monsters of a worse sort: those who were much like you but chose to exterminate all you knew anyway. Whatever the tribulation, at the end, only you remained of those who entered the crucible with you.

Surviving something like that changes you deeply. Despite the nightmares and trauma, you never broke. Perhaps you can’t break, now: all the soft, vulnerable spots in your psyche have been made hard. That hardening, however, makes it difficult to connect with others who can’t truly know what it’s like to have such an experience. You’ve probably turned to adventuring because there’s no longer a place for you in normal society. Somehow, fighting for your life in the darkness again and again is comforting. At least, now, you usually choose what you face.

Skill Proficiencies: Choose two from Perception, Stealth, or Survival
Tool Proficiencies: One type of artisan’s tools (used to make items needed for your survival)
Languages: The language of the enemy that pursued you
Equipment: A simple weapon (a rare example of a fully-functional item you salvaged), a set of artisan tools with which you’re proficient, slightly-damaged common clothes, and a belt pouch with 10 gp worth of assorted coins you’ve scavenged.

Feature: Vigilance

It served you well during your ordeal to keep an eye on your surroundings at all times, waiting for an attack from any direction, and you have not gotten over this wary anxiety. You have advantage on rolls to detect an ambush or otherwise avoid attempts to surprise you with an attack, even while theoretically safe (which can counter disadvantage from these conditions, such as being asleep). (Opponents have disadvantage on rolls to ambush you that target your Passive Perception.)

Variant Feature: Improvisation

You can generally scrounge materials to create equipment that is serviceable from the detritus of civilization. Given access to your artisan’s tools and some kind of raw materials that would make sense in the construction, during a short rest you can cobble together an item that will be functional, if flawed. The item tends to break whenever you roll a 1 using it (or an enemy rolls a 20, for armor other other defensive items), and, in the case of alchemical concoctions, does half its normal effect and expires in 24 hours.

Suggested Characteristics

Sole survivors tend to be damaged in a number of ways, but with a deep inner strength and conviction that saw them through.

d8 Personality Trait

1. The safest place for food is in my belly. I don’t know when I’ll get to eat again.
2. I don’t talk much. I guess I got out of the habit.
3. “Unflappable” is a good word for me: it takes a lot to make me flinch.
4. There are a few seemingly meaningless perks of civilization that I’m disproportionately excited about. I never thought I’d see them again.
5. People are neat (even people I don’t like). I’m just happy to be here, among the living.
6. I relate many things back to anecdotes about people I lost; uncomfortably so.
7. My humor is incredibly dark. If you can’t laugh, you’ll go crazy.
8. While I understand the concept of ownership, I have few qualms about walking off with practical items that I think might be necessary later that nobody else seems to be using.

d6 Ideal

1. Final Girl/Boy. When someone comes after you, you can only give them so much space before you have to push back. Hard. (Neutral)
2. Protector. I lost a lot of good friends. No one else should ever die like that again, especially not my new friends. (Good)
3. Reaver. There’s not necessarily anything wrong with what was done to me. I just need to be strong enough that it’s me on the other side, next time. (Evil)
4. Sheriff. What happened to me was only possible because my allies didn’t stand together and fight back. I hope to show that if we all work together, society is stronger than the monsters. (Lawful)
5. Survivor. At the end of the day, staying alive has its own meaning. Higher callings are all well and good, but every new sunrise you see is a victory. (Any)
6. Wildling. While I hated everything about being hunted, there was something free about just me and my friends against the darkness. Being back in society, I feel like all its rules are only there to make us vulnerable to further attacks. (Chaotic)

d6 Bond

1. I’m trying to find meaning in why these things happened to me. Is there ultimately a way to extract justice? Or was it just an evil I have to contend with?
2. The friends I lost have no one but me to carry out their legacies. I need to protect those they loved and enact their agendas.
3. I don’t think what happened to me was an isolated incident. I want to track down and stop similar things from happening again.
4. The scars I earned in my trials still hurt, and I worry that I might be somehow infected. Am I doomed to become that which pursued me?
5. I left something important behind in the place where I was hunted. Some day, I hope to reclaim it, but it is far and I am not prepared to revisit it, yet.
6. I liked who I was before. I was happier then. Perhaps, by reconnecting with people and places of my youth, I might someday recapture that feeling.

d6 Flaw

1. I can’t differentiate between the beings/nationality that attacked me, and have no belief that any of them can be other than monsters.
2. They mostly came at night. I’m obsessive about maintaining a watch and don’t sleep soundly even in the safest locations.
3. I’ll never fully trust anyone that hasn’t proven they’d die for me. Looking for ulterior motives and danger tends to alienate potential allies.
4. I’m too trusting of anyone that isn’t one of my tormentors, so used to everyone banding together against the greater threat.
5. Basically feral at this point, I have a hard time maintaining normal, emotional relationships. Everyone is either combat partner, risk, or threat.
6. Because of incidents where my enemies played dead, I’m dedicated to ensuring my opponents can’t get back up when my back is turned. It’s gruesome.