Inverted Puzzle Monster

[virgil]

We largely know how puzzle monsters work; from rakshasas to trolls to Gygaxian demi-liches. These are essentially monsters with a Steve-level weakness, usually borderline invincible until you use their kryptonite. What about a kind of inverse? The monster is perfectly vincible, but gets a buff in very limited circumstances (against a PC party); regeneration when in illumination less than bright light, damage reduction against natural weapons, attack bonuses when there's only one visible enemy, greater damage against targets without an armor bonus, etc.

How much does such a buff mess with CR?

[FatR]

If say, all fey creatures are vulnerable to iron (not some bullshit meteoric alchemical iron but your normal swords and arrows) and resistant to every other source of damage, would you consider them puzzle or reverse puzzle monsters?

[virgil]

I'd reflexively say reverse puzzle monster, but that situation is a little easier to set up than my examples/ideas. Not to say the question of how to assess the challenge of fairies with DR negated by iron/steel weapons.

[Kaelik]

If a Reverse Puzzle Monster exists, it will always and forever exist in a place where it's Reverse Puzzle exists, rendering it automatically a puzzle monster.

"Cut off the light shining on the monster" is not a different encounter from "Shine bright light on the monster." It's literally the same puzzle.

[virgil]

Always and forever? Some of the puzzles I'm positing are things like "armor bonus" and "have a friend in the room"; is such a monster mandated to only exist in bathrooms or with wall summoning powers, respectively? The benefit can be real and work against a number of opponents (darkness beast vs drow, for example), but prove to be unlikely to apply against a typical party to a significant extent.

[pragma]

I'm with Kaelik: Your inverse puzzle is still a puzzle.

A monster with a powerful buff that goes away when the PCs take the right actions is indistinguishable from a monster that receives a powerful nerf when the PCs take the right actions. The defining feature of both of these cases is that the monster becomes easier (possible?) to defeat if the PCs change their behavior or the environment in some non-obvious way.

I don't think that should stop you from designing your inverse puzzle monsters; I think there are ways to make the mechanics you've described into interesting encounters. I just don't think you've invented a concept that is meaningfully different than a puzzle monster.

[angelfromanotherpin]

There are obviously a lot of enemies which already qualify for this. Water elementals are more dangerous in the water. Trolls are more dangerous in closets. Fire giants are more dangerous in volcanoes. The CR system doesn't really compensate at all for contrived advantageous situations, you mostly just have to be aware of them.

It makes me think of combo-ing otherwise marginal effects together for advantage. Your hypothetical who gets an attack boost against armorless foes is going to be more reliable when teamed up with a rust monster, and your guy who doesn't regenerate in bright light wants to hang out with a Darkness SLA-slinging buddy.

[Eikre]

Combined arms are good tactical design. If the players get to see the cause and effect and then determine how they want to pull the thread, you're looking at a significant design upshot over the ordinary puzzle scenario of "Rumpelstiltskin is going to stab you in the dick until you say the magic word."

[virgil]

I just don't think you've invented a concept that is meaningfully different than a puzzle monster.

I didn't think I was coming up with anything novel, and certainly didn't intend to come off as thinking that. The primary question was help on figuring out CR for something like this. Here's an example...

Scapegrace
Size/Type: Small Outsider (Evil, Extraplanar, Lawful)
Hit Dice: 1d8 (4 hp)
Initiative: +1
Speed: 30 ft. (6 squares)
Armor Class: 15 (+1 size, +1 Dex, +3 natural), touch 12, flat-footed 14
Base Attack/Grapple: +1/-3
Full Attack: 2 claws +2 melee (1d2)
Space/Reach: 5 ft./5 ft.
Special Qualities: Darkvision 60 ft., Fast Healing 2, Immunity to fire and poison, Light Blindness, Resistance to acid 10 and cold 10, See in Darkness, Telepathy 100 ft.
Saves: Fort +2, Ref +3, Will +2
Abilities: Str 11, Dex 13, Con 10, Int 9, Wis 10, Cha 6
Skills: Climb +4, Craft (jewelry) +4, Hide +9, Listen +6, Move Silently +5, Search +4, Spot +6
Feats: Alertness
Environment: A lawful evil-aligned plane
Challenge Rating: 1/3 (??)

• Light Blindness (Ex) Abrupt exposure to bright light (such as sunlight or a daylight spell) blinds scapegrace for a round. On subsequent rounds, they are dazzled while operating in bright light.
  Fast Healing (Ex) A scapegrace heals only if it is not within 30' of a manufactured holy symbol (to any deity) or anything with a moderate (or greater) aura of good or chaos that has line of effect to it.

On the one hand, this creature is mechanically very similar to various monsters of CR 1/3, but most significantly has fast healing in limited circumstances.

[hogarth]

How much does such a buff mess with CR?

3.5E D&D says that favorable terrain for Team Monster will increase the CR of an encounter by 1. Obviously, that's a very rough guideline.