The Relentless monster trait can prevent a certain amount of damage from downing the creature; is the amount arbitrary?

Question

I'm reading a couple of entries in the Monster Manual of creatures with the Relentless ability; these include the Boar, Wereboar, and Giant Boar.

The ability reads along the lines of:

Relentless (Recharges after a Short or Long Rest). If the [creature] takes [number] damage or less that would reduce it to 0 hit points, it is reduced to 1 hit point instead.

To me, there doesn't seem to be an explanation for the amount of damage to avoid triggering the effect. This creates a problem when I want to give this ability to a custom creature.

Are there guidelines I'm missing? If there are not, how do I know what number to use there?

The creature(s) I'm homebrewing are Swine NPCs (SwinePCs) that possess this ability, and their final CR ranges from 1/4 to 5. I know I can just use the numbers from the Boar and Giant Boar for the same CR levels, but I'm stuck with the CR 5 creature.

Answer 1

While not exact, the data between the HP and the CR is linear enough to get an idea how Relentless would work on creatures of different CR.

Plugging the HP and CR values into anything that will give a linear fit like Wolfram gives a good estimation for other possible creatures: Wolfram gives the equation of \$6.43787 + 1.86982 \cdot CR\$.

Which when plugged give the following:

\begin{array}{llll} \text{Creature} & \text{CR} & \text{HP for Relentless} & \text{Estimate based on equation}\\ \hline \text{Boar} & 1/4 & 7 & 6.846155\\ \text{Giant Boar} & 2 & 10 & 10.11834\\ \text{Wereboar} & 4 & 14 & 13.85798\\ \end{array}

Plugging in for a CR 5 gives a value of 15.7278 which either 15 or 16 would seem appropriate for a CR 5 creature with Relentless.

Answer 2

Well, the Giant Boar is CR 2 and the damage is 10, and the Wereboar is CR 4, and the damage is 14, so you could estimate the difference between them and say you increase the damage by 2 for every CR increase, giving the CR 5 a damage of 16

I know it doesn't correlate with the boar damage, but it still works fairly well. Also, keep in mind the difference between CR 4 and CR 5 for HP is because it needs to take into account that at 5th level, most sword swingers are getting extra attack, and the spellcasters are getting 3rd level spells

Answer 3

How to reverse-engineer the amount

Examining the known data

Observe the following per each monster with the Relentless feature:

• The amount of damage to bypass Relentless equals an average roll for a certain number of d6's (see the "To Bypass" column of the following table).
• The Dungeon Master's Guide recommends¹ a particular increase to the effective HP of a monster with Relentless based on the creature's CR, and this increase also equals an average roll of d6's (see the "Effective HP" column of the following table).

$$\begin{array}{lcrr} \text{Monster} & \text{CR} & \text{Effective HP} & \text{To Bypass}\\ \hline \text{Boar} & 1/4 & +7 (2d6) & 7 (2d6)\\ \text{Giant Boar} & 2 & +7 (2d6) & 10 (3d6)\\ \text{Wereboar} & 4 & +7 (2d6) & 14 (4d6)\\ \text{Firemane Angel}^2 & 5 & +14 (4d6) & 21 (6d6)\\ \end{array}$$

Finding a simple pattern

We can make some observations about the above table, combined with some reasonable speculation:

• The number of d6's for the effective HP boost is 2 fewer than the number of d6's for the damage to bypass (minimum of 2d6). This observation fits the data, but so do other possible explanations. I'm going to stick with it, though, because there are plenty of other game features that have edge cases at lower levels or CR's, so it's not unexpected.
• The pattern speculated above is consistent for CR's with known example monsters with the effective HP scale given in the DMG (reproduced in the following table).
• The missing bypass numbers for CR's without examples can be deduced from the pattern (see the figures in "To Bypass" marked with \$\leftarrow\$ in the following table, which are the only speculative figures; everything else is sourced above).
• The CR range scale from the DMG matches the 4 tiers of player characters by level.

$$\begin{array}{lcrrl} \text{CR Range} & \text{Player Tier} & \text{Effective HP} & \text{To Bypass}\\ \hline \le 4 & 1 & +7 (2d6) & \le 14 (4d6)\\ 5-10 & 2 & +14 (4d6) & 21 (6d6)\\ 11-16 & 3 & +21 (6d6) & 28 (8d6) & \leftarrow\\ \ge 17 & 4 & +28 (8d6) & 35 (10d6) & \leftarrow\\ \end{array}$$

Then the figures in the above table are reasonable because 1) they fit all the known data (the speculative 28 fits the known 28 in the following row) and 2) they are straightforward to calculate based on a simple formula (which we would expect based on 5e's design principles).

Conclusion

Based on the above, I propose that you use the figures of 14, 21, 28, and 35 as the SwinePC advances through the 4 tiers of play.

Likewise, for customizing a monster at the higher CR's, use the corresponding numbers.

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Footnotes

1. See Step 13 of Creating a Monster Stat Block and the corresponding Monster Features table.
2. The firemane angel (warning: paywall) appears in Guildmaster's Guide to Ravnica, which is a first-party Wizards of the Coast publication. It's obviously not swine, but it's the only other data point we have and the math fits, so we shouldn't ignore it.

Answer 4

EDIT: This was written prior to understanding the Relentless completely, as this is only for one attack its not as bad, but still non-linear for what attacks will be prevented.

The actual challenge presented by an escalating Relentless number is not actually linear.

The amount of damage a sword does per attack does not generally increase very quickly. If this number is set too high, it effectively makes the creature immune to death without expending a resource of some sort.

A level 1 party with a 16 str character with a longsword can expect to do a ceiling of about 13 damage with said longsword. That makes the wereboar extremely intimidating as it is essentially indestructible to said longsword attacks. (Abilities spent on this as resources such as spells, battle mastery points, rages and so on can bump this number)

Getting to level 8 essentially just bumps this by 2-3 points assuming no magic items.

So just keep this in mind when scaling your critter. That the 1/4 CR Relentless is very easy to bypass, the wereboar more limiting as to which party member can finish it off and higher basically requires gaining dice or extra damage bonuses.

At level 5 or 8 there are many of these available to the spellcasters/roguelike strikers, but this may reduce the amount of the party able to finish off the opponent.

Answer 5

A simple way of looking at it is that you are effectively adding X-1 HP. If the monster was at 1 HP, and it is dealt X-1 damage, then it stays at 1 HP. The DMG tables will tell you if this "extra HP" would suggest a higher CR.