Enhancing/Advancing monsters, and ECL/CR considerations

[Greenfield]

This came up in our game.

You want to enhance a monster by giving it an energy resistance or immunity.

How does that play into the Challenge Rating?

Yeah, yeah, I know, CR system is somewhere between deranged, disfunctional and just plain broken, but humor me here.

I want to give a critter a 5 point Electrical or Fire resistance. Does that bump the CR?

10 point resistance? CR Adjustment? If so, by how much?

Energy immunity?

The normal rules for advancing monsters adds to their HP and attack bonuses. It may or may not add to their feats and/or skills.

Defenses might bump if they are the type of thing that uses gear, like armor or magic items, but that doesn't include everything, not by a long way.

And special abilities, such as exotic attack modes, energy resistances etc are never mentioned anywhere at all.

So aside from the specific question which, as I said, came up inplay, what would be the general rules for this? Are they house rules, or written down somewhere?

 

[Tinker]

The are created guidelines for homebrew creatures. I'm AFB so can't remember where just now.
Adding a single ability like energy resistance would depend on the overall power level of the creature and, if you're creating it for a specific encounter, the party's likely attack forms. Fire resistance 5 on a cr 1 creature when the party sorcerer has burning hands, worth a bump. On a cr 10 creature or vs a necromancer, not so much.

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[Tinker]

*there are cr guidelines. Sorry

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[ElectricDragon]

I try to find a template that does that (and not much else if possible) and use the CR adjustments there. For instance: for CR, Fiendish Creature Template says: HD 3 or less, as base creature; HD 4 to 7, as base creature +1; HD 8 or more, as base creature +2.

The template provides: cold and fire resistance; it starts at 5 and goes up to 10 at 8 HD; darkvision; and DR at 4th level and beyond.
As you can see, CR does not change from resistance of 5 in two common energies, but goes up when DR is added and again when resistance becomes 10
The Celestial Creature Template does much the same, but with 3 resistances instead of two.

I should note that I consider cold, fire, and electricity common energies; acid, positive and negative uncommon energies; and sonic and force rare energies; as far as CR considerations. YMMV.

 

[Celebrim]

Depends very much on the monster. Giving a troll or a frost giant fire resistance matters much more than giving the average monster fire resistance.

 

[ElectricDragon]

Oh yes, adding on further resistances and adding resistance to regeneration; those mean more in context. Is that what you really were asking?

 

[Grogg of the North]

The other thing to consider is if you're giving a creature Fire Resistance and if the party uses a lot of fire. Yes, giving the monster fire resistance may explain why it lives in the fiery cave but if the party only has an ice mage then ... who cares about the CR?