Changing monster abilities. Change CR?

[Herzog]

I'm planning to run an encounter with a Gibbering Mouther, but for campaign specific motiviations I want to change some of it's abilities.

I plan on removing it's Ground Manipulation ability, and replacing it with Alternate Form (I will start the encounter with the Gibbering Mouther impersonating another monster) and Telepathy (to allow it to communicate with one specific PC whithout the rest of the party overhearing)

The Alternate Form will allow the Gibbering Mouther some extra speed while in the form selected. It will also add an extra level of horror (I expect) when a normal humanoid suddenly sprouts extra eyes and mouths at various parts of its body, starts 'gibbering' and then 'melts' into the Gibbering Mouther's true form.

All this of course for full story effect.
However, I have never run a Gibbering Mouther in its 'normal' form, and have no idea what the impact is going to be of the changes I made. Should I raise the CR because of the Alternate Form ability? or lower it for removing the Ground Manipulation ability?
Any thoughts?

 

[Shin Okada]

It's hard to estimate the value of different monster special abilities. But as both of them are not direct attack abilities, that exchange may not affect much on the formidableness of the Mouther as an opponent.

But as Alternate Form gives physical ability scores and other physical features to the Mouther, depending on the selection of forms, it may actually become a very strong ability for combat.

So, what kind of forms the mouther can take will be very important.

 

[Herzog]

mmm. that's right. forgot about that.

I actually just want to use the alternate form as an impersonation, allowing a mouther to get a surprise round when it attacks (more similar to disguise self) not as a way to really 'become' another creature.

Does that lower it's CR in your opinion?

 

[freyar]

Just say that the Alternate Form can only take some specific humanoid form. That's not going to improve the physical stats, really. Or you could just use Change Shape, instead (physical stats don't change).

Anyway, I'd possibly drop the CR to 4 they way you are talking about using the disguise. The ground manipulation isn't bad, and the time to change forms also takes a Standard Action, I think.

 

[Herzog]

I'm thinking about requiring a standard action to HOLD the shape (given the nature of the creature), allowing it to loose shape as a free action.

Basically, I will have replaced an attack-improving ability with an ability that adds roleplay and atmosphere, and may result in a surprise round.

I think I'll go with the lower CR of 4. Thank you.