When do monsters not have average hp?

Question

The Monster Manual (7) says:

A monster's hit points are presented both as a die expression and as an average number. For example, a monster with 2d8 hit points has 9 hit points on average (2 x 4 1/2).

The Dungeon Master's Guide (276) says:

A monster's hit points have a direct bearing on its challenge rating, and vice versa. You can determine a monster's hit points in one of two ways...
Assign Hit Dice. Alternatively, you can assign a number of Hit Dice to a monster, then calculate its average hit points.

Both of these sources seem to indicate that monster hp are 'calculated' by taking the average hp that would be rolled on the monster's HD.

However, published monster hp are not always listed as being the average value given their HD. For example, in the adventure Tales of the Yawning Portal, we find the following monsters:

Drow. HD3d8. Hp. 13. The average of 3d8 is 13 (3*4.5 = 13.5, rounds down).

Drow Elite Warrior. HD11d8+22. Hp. 71. The average of 11d8+22 is 71 (11*4.5 + 22 = 49.5 + 22 = 71.5, rounds down).

Drow Commander. HD11d8+22. Hp. 110. The maximum of 11d8+22 is 110 (11*8 + 22 = 88 + 22 = 110).

Note that the drow elite warrior and drow commander have the same HD, but the Elite Warrior has average hp and the drow commander has the maximum possible for its HD.

It certainly seems intuitive that 'leader types' should have more hp than average, but is this design principle ever stated or explained in the rules? Are leader-types with non-average hp always given the maximum amount, or do they sometimes have more than average, and sometimes more than the maximum?

Answer 1

In Tales from the Yawning Portal, in the Stinking Caverns (#20) I see this text:

Currently in this place (at the least) are six drow and four drow elite warriors, one of which is a female commander who has 110 hit points. The commander has a +2 shortsword, a +1 hand crossbow, and three magic bolts, as follows...

In the DMG, under Creating a Monster Stat Block (step 8) I see your quoted DMG text and also the following (emphasis mine):

A monster typically has average hit points based on its Hit Dice. For example, a creature with 5d8 + 5 hit points has an average of 27 hit points (5 × 4.5 + 5).

So while a monster typically has the average hit points dictated by its hit die, it doesn't have to. And it's pretty clear that the Commander in Tales... is a Drow Elite Warrior that's been buffed up a bit, both by giving it the max HP and by giving it some extra items other Elite Warriors don't have.

Your title question is When do monsters not have average hp? There's no hard and fast rule here. For your specific example, while we don't have insight into the process by which the Commander was created, it seems possible that the intent was to create another tier of Drow that was slightly tougher than the other Drow in this part of the adventure, and that the designer of the adventure decided to do this by giving the Drow non-typical items and HP.

Other instances will be at the DM or the designer's discretion.

Answer 2

Whenever the adventure writer decides so

First off, it is up to you if you roll hp for a monster, or use the average number that is provided for simplicity and convenience (just as you can roll for damage or use the average provided). However, there are cases where monsters are given an explicit hp number that differs from the average.

Below are some examples from published adventures where the hit points deviate from the expected average. You can have higher hp, lower hp, even hp exceeding the normal maximum and additional abilities. When is that commonly done?

It seems to be pretty common to give major leaders or powerful individuals maximum or near maximum hit points. This makes them last longer so they will deliver a more memorable fight, and does not cause overhead like adding abilities, although for some leaders this is also done, making them even more impressive. In Tomb of Annihilation we get:

• A pterafolk leader with 40 hp instead of 26, near the maximum 44
• An aarakocra leader with 31 hp instead of 13 and even above the maximum 24, higher intelligence, wisdom, additional skills and spellcasting like a 5th level caster (which is enough to increase the CR to 2)
• A giant snapping turtle called King Toba with the maximum 120 hit points instead of 75, and advantage on saves against spells and magical effects
• A tyrannosaurus rex called the King of Feathers with 200 hit points instead of 136, near the maximal 208, and with other special traits such as seeing invisible creatures and legendary resistance
• The Wild Beyond the Witchlight has a 40 x 8 foot rug of smothering with the maximum 60 hp instead of 33.

It also is common to give lower hit points to weak opponents, for example very young or old ones that are infirm. In Tomb of Annihilation we get:

• Gondolo, a scout with 13 hp (instead of 16), portrayed as an incompetent fortune hunter
• A fledgling axe beak with 6 hp (instead of 19) and no effective attacks
• Elderly pterafolk with 13 hp (instead of 26)

Lastly, when the size of creature changes, then following page 7 of the Monster Manual their hit dice change, and this can affect hit points even when they are the average for the new adjusted size. In The Wild Beyond the Witchlight, we get:

• Sowpig, a small ghoul with 17 (5d6) hit points instead of 22 (5d8)
• Small animated armor with 27 (6d6+8) hp instead of 33 (6d8+6)

The DMG also provides this advice on page 273 about modifying existing monster stat blocks:

Adapting a stat block is far less time-consuming than creating one from scratch, and there are changes you can make to an existing monster that have no effect on its challenge rating, such as swapping languages, changing its alignment, or adding special senses. However, once you change the creature's offensive or defensive ability, such as its hit points or damage, its challenge rating might need to change, as shown later

In most cases above where only the hit points were changed, the challenge rating was not changed, even for maximum hit points. For example, King Toba with its 120 hp has the same CR as a normal giant snapping turtle with 75, which normally would increase the defensive challenge rating from 1 to 4, and the overall challenge rating by at least one.

Given how inexact CRs are to begin with, it seems it is considered OK to simply keep the CR as is when picking a number like the maximum that is a possible outcome of a roll for the creature's hit dice. A CR adjustment is done in exceptional cases, like that of the aarakocra leader who also has multiple caster levels.

Answer 3

Monsters have non-average HP any time you want them to.

The average HP listing, like average damage listings, is just a convenience feature. As DM, it's often not worth your time to roll randomized HP values for every monster your PCs fight, so you can just use the average to make it faster.

But you can roll for it, or pick a value within the range, any time you want a monster to be unusual. Maximized hit points can make a boss monster a bit more challenging, as in the case of the drow crew you mentioned, or it can just be a way to make sure your players haven't just memorized how much HP a red dragon has.

...even mid-combat

A secret side benefit of the HP randomization is that it gives you (as DM) the option to slightly alter a monster's HP for dramatic impact.

For example, if the paladin swings in with a massive critical smite and leaves the monster with 3 HP, you may want to waive the few HP and have it die from the big hit instead of lingering on with a handful of hit points, waiting for somebody to drop it with a cantrip. Or contrariwise, if your PCs do really well, you might want to have the monster survive with a couple HP left instead of going down right before it gets to do its One Cool Move.

If you had the monster with average HP to begin with, you can justify the change by retroactively deciding the monster had slightly less or slightly more HP than average from the start. It's not really cheating, since it could have had that much HP to begin with, you're just nudging the story a bit.