Reactive monster - help me build it?

[Heliomance]

So I was watching the Legend of Korra, specifically the season 2 bit where they're exploring the spirit world, and I got to thinking that spirits might make a rather interesting surprise to drop at players. In particular, the way they react and change depending on your state of mind.

I want to try and build an enemy that reacts to how the party approaches it. If they attack, it becomes aggressive and gets combat boosts. If a Barbarian Rages at it, it grows bigger and gets stronger. If the party is trying to be sneaky, it gets stealth bonuses, Darkstalker, and Hide in Plain Sight. If they don't attack, and approach it in good faith, it's automatically friendly. If they negotiate in bad faith, it gets bluff bonuses, and will try and backstab them at the earliest opportunity.

I'm thinking a Fey base for it, maybe CR7-ish? Let's brainstorm this, see what falls out.

[gooddragon1]

So I was watching the Legend of Korra, specifically the season 2 bit where they're exploring the spirit world, and I got to thinking that spirits might make a rather interesting surprise to drop at players. In particular, the way they react and change depending on your state of mind.

I want to try and build an enemy that reacts to how the party approaches it. If they attack, it becomes aggressive and gets combat boosts. If a Barbarian Rages at it, it grows bigger and gets stronger. If the party is trying to be sneaky, it gets stealth bonuses, Darkstalker, and Hide in Plain Sight. If they don't attack, and approach it in good faith, it's automatically friendly. If they negotiate in bad faith, it gets bluff bonuses, and will try and backstab them at the earliest opportunity.

I'm thinking a Fey base for it, maybe CR7-ish? Let's brainstorm this, see what falls out.

Sounds like you might want to take a sub template idea similar to the rogue special ability class feature. Activation might be a problem because if the party is sneaky how would it know they were there? Maybe you could simply have it react to the way in which damage is dealt. If it's magical attacks then it gains the ability to use magical abilities. If it's sneak attack damage then it gains stealthy assassin like abilities. If it's straight up strength damage then it changes to that. Also, if it were to react like that, should it change immediately when damaged? Could it potentially gain multiple types of abilities by not changing but adding? Or perhaps it could choose to change abilities if struck by a different category? Will it survive the initial attack... and for now, how many categories do you want to go for?

[Heliomance]

Sounds like you might want to take a sub template idea similar to the rogue special ability class feature. Activation might be a problem because if the party is sneaky how would it know they were there? Maybe you could simply have it react to the way in which damage is dealt. If it's magical attacks then it gains the ability to use magical abilities. If it's sneak attack damage then it gains stealthy assassin like abilities. If it's straight up strength damage then it changes to that. Also, if it were to react like that, should it change immediately when damaged? Could it potentially gain multiple types of abilities by not changing but adding? Or perhaps it could choose to change abilities if struck by a different category? Will it survive the initial attack... and for now, how many categories do you want to go for?

The spirits in LoK do it by feeling and reacting to the other person's emotional state, so I figured probably something akin to Mindsight would be needed.

[gooddragon1]

The spirits in LoK do it by feeling and reacting to the other person's emotional state, so I figured probably something akin to Mindsight would be needed.

Psionics to the rescue: Empathy (http://www.d20srd.org/srd/psionic/powers/empathy.htm)

Doesn't tell you where they are like mindsight would, no save or power resistance, detects surface emotions. You could just play this straight up and not have it cater to their class and abilities but rather their exact emotional states. You could also go with a variant form of it too.

[ezkajii]

I think that the empathy power is really pretty well perfect for this, flavor-wise, and then you would just need to come up with the exact methods it uses to respond to what it's perceiving. The 'trouble' is, it's going to rely heavily on roleplaying and/or DM adjudication with regard to what the creature can detect and so forth. For a crunchier version, I would recommend making the creature have a series of abilities that can react simultaneously as appropriate. For instance, I might do something along the lines of the following:
Emulate Alignment (Ex): A [creature]'s alignment is neutral unless a creature or group of creatures whose alignment matches and HD total exceeds the [creature]'s own is within 100ft. In this case, the creature's alignment changes to match this alignment representation. It returns to normal when this condition is no longer met.
Reflect the Hidden (Su): Whenever a creature within 100ft of a [creature] succeeds on a Hide or Move Silently check against it, the [creature] gains a +10 circumstance bonus to both Hide and Move Silently. It attempts to become hidden to the best of its ability at the first opportunity, so long as at least one creature is still hiding from it when that opportunity arises.
Automatic Spell Stowaway (Su): Any time a spell that targets individual creatures is cast within 100ft of a [creature], it is also targeted by that spell. If the [creature] was already a target of the spell, this ability has no effect.
Variable Damage Reduction (Su): When a [creature] is dealt bludgeoning, piercing, or slashing damage in combat, it immediately (after the attack resolves) gains DR/- equal to 1/5 the damage it took. This DR value changes each time it is struck; if one minute passes without taking damage of any kind (including ability damage, fire, etc.), the DR resets to zero.
Adaptive Energy Immunity (Su): If a [creature] takes energy damage of any kind, it immediately gains immunity to that type of energy. The creature can have only one energy immunity at a time through use of this ability.
Breath Weapon (Su): A [creature] only possesses a breath weapon if it also possesses an energy immunity through its Adaptive Energy Immunity ability; the breath weapon deals [HD]d6 damage of the same energy type as the immunity.

and so on. Also I would personally do it as an incorporeal fey, targeting CR 11ish, if you do it the more detailed way i sampled here. Just the sheer number of adaptations would make it probably impractical to try to squeeze into a CR 7.