Improving a monster with an original ability and CR (my players stay out!)

[Olive]

So if I want to add an ability to a CR12 Monster, how much does it increase the CR of that monster.

I want to make this creature have a song that allows it to cast Bestow Curse 3/day. It will affect all creatures that can hear it, and will be consdiered a Spell-like ability.

I'm not sure what I need to take into consideration when working out the new CR.

 

[The Souljourner]

Bestow Curse on everyone that can hear it!? Holy crap is that nasty! You realize the only reason bestow curse is at all balanced is because it's a touch spell, right? I mean, dayum! Not only are you giving it a big range, it also affects everyone in hearing range. Sheesh!

I think that raises the CR by at least 2 if not more. Are you sure that's what you want the ability to be? A -4 penalty to most rolls for everyone who hears this, or a -6 to an ability score?

Geez, that's really rough.

-The Souljourner

 

[Olive]

I know. That's why I as asking. I was thinkign of making it like the charm person song that the siren has (MM2) but it's a demon and I've already given it a cursespewing sword, so I thought I'd make it bestow curse.

How about 1/day?

 

[Elder-Basilisk]

Whether it's 1/day or 3/day really makes little difference as the creature is only likely to face the PCs once.

If you want a damaging song for the creature, I'd recommend some kind of fear aura. That seems appropriate to a demon. If it just makes PCs who fail a will save shaken, I'd charge the creature one or two feats for the ability (one if it already has a fear aura like the Ossyluth does; two if it doesn't). If your PCs use Hero's Feast on a regular basis, the song will have no effect, so don't adjust anything since basically all you're doing is adding flavor to the effect of "he tries something that doesn't work and then does this."

Alternatively, you could treat it as Prayer: +1 for the creature and -1 for the players. Remove a spell like ability it has at 3rd level and stick prayer in its place. If it has enough hit dice, use one of its feats for Quicken Spell Like ability and you're ready to go.

 

[Garnfellow]

To my mind, the gold standard for this sort of analysis is FFG's Monster's Handbook. Using those guidelines, the first thing to do is to quantify the power of this ability -- if it were a spell, what level would it be? This is the trickiest part of the process, since assigning a level to a new spell usually comes down to some judgement calls.

This monster's ability is a really nasty, /permanent/ effect. The closest analogue I can think of is the 5th-level symbol of pain, which approxmates only only one application of bestow curse (-4 to rolls) on a temporary basis.

So let's say we start with base spell level of 5, add +2 levels to make the effect permanent, and another +1 level to reflect the multiple applications of the spell (-6 to one ability score, etc.) An 8th level spell? That looks about right, especially next to symbol of death.

Once we've got this down, the next step is to assign the new ability its own CR rating, just as if it were a separate monster. The Monster's Handbook suggests that adding a new spell-like ability that approximates an 8th-level spell is much like adding a second 12 CR monster to an encounter.

Although you call this a spell-like ability, your description of this as a song makes it sound more like a supernatural ability (which can't be disrupted). Making this spell-like ability a magical ability with 30 ft range would increase the CR by another +1, for a total CR of 13.

Adding this new CR 13 ability, then, to your original CR 12 demon is like setting up an encounter with 1 CR 12 monster and 1 CR 13 monster.

2 CR 12 creatures = EL 14; 1 CR 12 creature + 1 CR 14 creature = EL 15, so the adjusted CR for your monster would be somewhere between 14 and 15 (14.53, to be more precise).

Hope this helps!