Q: Improving Monster HD and Caster Level

[rushlight]

Alright, so I'm improving a monster for my party to face, and I'm increasing it's HD. I don't want to give away too much info here (since they are probably spying!) but here's the question:

If the creature has a Supernatural ability that functions as a spell, and the creature (currently HD 11) has a Caster Level of 13 for the Supernatural ability - what happens to CL when I increase the HD to say, 33? This will make it around CR 19-20, and it's for a 19th level party. The original CR was 13.

Thanks!

 

[Urbannen]

Extrapolating from Savage Species, it looks like the caster level would increase to 35.

Savage Species section on "monsters as classes" usually states that a monster's caster level for its spell-like abilites is equal to monster class levels + x. At the end of the normal progression, the monster has the normal caster level of its kind, in this case HD + 2. If you are advancing a monster strictly with HD and not character class levels, that's the same as adding levels in the monster class on a 1:1 basis, monster class level to HD. The formula should still hold: caster level = HD + 2.

 

[Garnfellow]

I'm not sure if there's better or more explicit information in the 3.5 MM, but I haven't found many definitive 3.0 rulings on this and similar issues when advancing mosters.

Increasing the caster level by +1 for every +1 HD makes a certain amount of sense. If no caster level is given for either a supernatural or spell-like ability, the default caster level is the monster's HD.

An alternative approach, based on the way SR appears to increase when advancing monsters, is to add +1 to the caster level for every +1 in challenge rating. This is generally a more conservative approach, so your CR 13, Caster Level 13 (is this a coincidence?) creature advanced to a CR 19 monster would use its supernatural ability as a 19th level caster.

In the absence of better guidelines, either approach is feasible.