Monster caster/manifester level

[The Vorpal Tribble]

Ok, I've always had it in my head how a monster's caster level for spell-like abilities should progress as you add on class levels but can't seem to find the exact rulings.

Say you have a marilith with 16 HD and a caster level of 16. If you add on class levels does the caster level rise with it?

How about by HD, does the caster level rise then?

[Djinn_in_Tonic]

I believe the caster level rises with either, as it makes sense to improve natural casting when advancing hit dice, and, looking at the Rakshasa's "As Player Character" section, it advances if the Rakshasa takes Sorcerer levels.

So both advance casting, by my best judgment.

[The Vorpal Tribble]

Well, what I'm looking for is a quote so I know I'm not just winging it.

[Djinn_in_Tonic]

Well, what I'm looking for is a quote so I know I'm not just winging it.

Sadly, the Rakshasa's entry is the closest thing to a quote I could find in any of the rulebooks I have access to right now. Savage Species might mention it, but I don't have that book handy at the moment.

[The Vorpal Tribble]

Yeah, at least it gives me a basis to point at and go 'Well, HE did it'.

[tyckspoon]

I'm not aware of any general rules that explicitly tie SLA caster level or inherent spellcasting progression to HD (there is a rule that racial spellcasting + the same PC class combine, so dragons and rakshasas and couatls can advance their casting with Sorc levels, but nothing on racial HD.) So, barring specific text in the monster that does so, adding either HD or class levels to a monster will do nothing to their SLAs; a 48 HD Marilith still uses its SLAs at CL 16, and a 27 HD Couatl still only has casting of a 9th level Sorcerer.