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I hope this question respects the site standards. I think that my question is quite specific and not too vague or opinion based. If not, I apologize in advance. I am creating an homebrew monster and I don't know how to properly calculate the CR following the instructions on the DM manual. The stats are as follows.

Retuning String

Medium monstrosity, unaligned


Armor Class 10 (natural armor)

Hit Points 148 (33d8)

Speed 0 ft., fly 30 ft. (hover)


STR DEX CON INT WIS CHA
1 (-5) 10 (+0) 10 (+0) 1 (-5) 10 (+0) 1 (-5)

Saving Throws: Con +?, Wis +?

Damage Immunities: poison, psychic; bludgeoning, piercing, and slashing from nonmagical attacks

Condition Immunities: all conditions but incapacitated, invisible and stunned

Senses: blindsight 120 ft. (blind beyond this radius), passive Perception ?

Languages: --

Challenge: ? (? XP)


Immutable Existence. The string is immune to any spell or effect that would alter its form or send it to another plane of existence.

Legendary Resistance (3/Day). If the string fails a saving throw, it can choose to succeed instead.

Limited Magic Immunity. The string is immune to spells of 6th level or lower unless it wishes to be affected. It has advantage on saving throws against all other spells and magical effects, and spell attacks made against it have disadvantage.

Pure Energy Entity. The string can move through other creatures and objects as if they were difficult terrain. Each creature it moves through takes 5 (1d10) force damage; no creature can take this damage more than once per turn. The string takes 5 (1d10) force damage if it ends its turn inside an object.

Retune. Whenever the string is subjected to damage, roll a d6. On a 1 to 3, the string takes no damage and regains a number of hit points equal to half the damage dealt. If the string takes damage from multiple sources, roll a separate d6 for each source of damage.

Unusual Nature. The string doesn't require air, food, drink, or sleep.

Actions

Harmful Frequency. Each creature within 30 ft. of the string takes 28 (8d6) damage of one of the following types (random choice): acid, cold, fire, force, lightning, necrotic, radiant, or thunder.

Healing Frequency (3/Day). Each creature within 30 ft. of the string regains 60 hit points and is freed from any curse, disease, poison, blindness, or deafness.

Legendary Actions

The string can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The string regains spent legendary actions at the start of its turn.

Harmful Frequency. The string uses its Harmful Frequency action.

Move. The string moves up to its speed.

Teleport (Costs 3 Actions). The string magically teleports, along with any equipment it is wearing or carrying, up to 30 feet to an unoccupied space it can see.


My specific question: how should I evaluate the Retune trait in order to compute the CR? Maybe I have to increase the effective AC by 4? Or double the effective HP?

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  • \$\begingroup\$ I suggest just showing the "retune" trait, as it is the only relevant part. \$\endgroup\$
    – User 23415
    Commented Apr 9 at 20:44
  • \$\begingroup\$ @User23415 given the unusual nature of the ability, how it interacts with the others might be important \$\endgroup\$
    – SeriousBri
    Commented Apr 9 at 20:49
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    \$\begingroup\$ Also, I suggest adding a Con save to Harmful Frequency. Alternatively, you could make each potential damage type target a different save. \$\endgroup\$
    – User 23415
    Commented Apr 9 at 21:01
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    \$\begingroup\$ Pure Energy Entity doesn't state what happens when the string ends its turn inside a creature. \$\endgroup\$
    – User 23415
    Commented Apr 9 at 21:04
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    \$\begingroup\$ @Leonardo if you want more in-depth suggestions, I'd suggest asking a [homebrew-review] question about this creature. \$\endgroup\$
    – User 23415
    Commented Apr 9 at 21:06

1 Answer 1

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Statistically, this is quadrupling the creature’s hit points.

Suppose the creature takes \$X\$ damage. There is a 50% probability that it takes \$X\$ damage and a 50% probability that it heals \$\frac{X}{2}\$ hit points. Thus the expected hit point reduction from \$X\$ damage is:

$$0.5X - 0.5(0.5X) = 0.25X$$

So for the purposes of calculating defensive CR, use 4 x 148 = 592 for its hit points since on average, you need to deal 592 points of damage to actually reduce its hit points by 148.

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    \$\begingroup\$ I'd note that this makes fights more swingy (some will end up trivial, some unbelievably, frustratingly, long), and is likely to frustrate your players, especially those with fewer, higher damage attacks (e.g. Rogues). I'd strongly recommend, at the very least, not including the healing component; doing more damage should not be worse that doing less. \$\endgroup\$ Commented Apr 10 at 22:08
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    \$\begingroup\$ That part bothered me too. "Roll damage. Great roll, this things heals for half of it. Roll damage, bad roll. This thing is damaged for that bad roll." Personally, I'd make a chance of "Chance to negate attack, doesn't heal from it" for non magic attacks one ability. And healing from attacks something different, like "This things feeds on magic attacks level 6 or less (heals from them for 50% damage100% of the time) but doesn't have advantage on spells of level 7 and above" \$\endgroup\$
    – jo1storm
    Commented Apr 11 at 9:41

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