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?
Best Answer
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.