The particular case in question involves a popular adventure path. Read the next spoiler to find out which one. If you dare.
These spoilers deal with the Curse of the Crimson Throne.
Ilesoa's Furies from the Crown of Fangs are Erinyes with 9 additional hit dice. Remember they are built with 3.5 rules.
If I add 9 additional hit dice to an erinyes without increasing her charisma score will her spell-like ability save DCs increase?
I ask because in a Paizo published D&D 3.5 adventure there is an 18 hit dice erinyes and all of her spell-like ability save DCs are increased by 5.
She has no feats or equipment that impact these DCs. If I advance a monster by adding hit dice will it affect their spell-like ability save DCs?
It wouldn't.
The saving throw (if any) against a spell-like ability is 10 + the level of the spell the ability resembles or duplicates + the creature's Charisma modifier.
Though the increased hit dice would increase the caster level of the spell-likes.
If no caster level is specified, the caster level is equal to the creature's Hit Dice.
Hmm. Interesting, the caster level on this particular erinyes is unchanged, both her's and an unadvanced erinyes are level 12.
I suppose the additional 5 points on the save DCs is a bit of fudging on Paizo's part. I don't blame them, I do it myself.
Did her Cha increase by 10 or so? Adding HD can be accompanied by increasing ability scores faster than at adding class levels.
Her charisma is unchanged. Both her's and an unadvanced erinyes are 20.
Phew. I checked 3.5's monster manual and it has this line on page 300:
Calculating Saving Throw DCs
The formula for save DCs is: 10 + 1/2 the creature's Hit Dice + relevant ability score modifier
So +9 HD results in +4 or +5 DC.
Phew. I checked 3.5's monster manual and it has this line on page 300:
So +9 HD results in +4 or +5 DC.Calculating Saving Throw DCs
The formula for save DCs is: 10 + 1/2 the creature's Hit Dice + relevant ability score modifier
That is for abilities that don't duplicate spells. Even in 3.5, spell-like abilities had a different formula:
For creatures with spell-like abilities, a designated caster level defines how difficult it is to dispel their spell-like effects and to define any level-dependent variables (such as range and duration) the abilities might have. The creature’s caster level never affects which spell-like abilities the creature has; sometimes the given caster level is lower than the level a spellcasting character would need to cast the spell of the same name. If no caster level is specified, the caster level is equal to the creature’s Hit Dice. The saving throw (if any) against a spell-like ability is:
10 + the level of the spell the ability resembles or duplicates + the creature’s Cha modifier.
Thanks Sheepish Eidolon. That was a good find.
The section you quoted concerns creating monsters rather than advancing them, but I think it could be where the +5 is coming from. In that section it gives a list of special attacks and spell-like abilities is on that list, then it gives the hit dice version for calculating the DCs.
They also give the same formula on page 6.
No it doesn't work. The 18 hit dice advanced erinyes has a 20 charisma just like an un-advanced erinyes, and her spell like abilities are:
Spell-Like Abilities (CL 12th)
At will—greater teleport (self plus 50 pounds of objects only), charm monster (DC 24), minor image (DC 22), unholy blight (DC 24)
If Paizo were following the formula 10 + 1/2 the creature's Hit Dice + relevant ability score modifier then all of the spell-like abilities would have the same DC.
Looks like it will just remain a mystery. ;)
Likely, the reason for the increase in DC is this:
Compare the monster’s improved attack bonus, saving throw bonuses, and any DCs of its special abilities from the HD increase to typical characters of the appropriate level and adjust the CR accordingly.
Just instead of adjusting the CR to fit the DCs, they made the DCs fit the CR.