The souldrinker prestige class does not have a spellcasting prerequisite, however many of the abilities refer to "her spellcasting ability modifier"
What ability score would you use if you entered the prestige class with no spellcasting ability?
"If in doubt, use charisma" does seem like a safe default, and there is plenty of precedent.
It won't be an official answer, but I'll look back at the draft of the PrC and see if I can provide an answer as to the intent (or a full answer if I simply left out some wording).
Spells per Day: At the indicated levels, a souldrinker gains new spells per day as if she had also gained a level in a spellcasting class she belonged to before adding the prestige class. She does not, however, gain other benefits a character of that class would have gained, except for additional spells per day, spells known (if she is a spontaneous spellcaster), and an increased effective level of spellcasting. If a character had more than one spellcasting class before becoming an souldrinker, she must decide to which class she adds the new level for the purposes of determining spells per day.
I take this to mean that it could be any spell casting class. The "spellcasting ability modifier" would/could be whatever spellcasting class she belonged to. If multiple classes apply, one should be chosen as the primary class and once the decision is made it'd be permanent.
It looks to me as if the class simply should have required a caster level of some sort, and it was omitted due to an error.
I take this to mean that it could be any spell casting class. The "spellcasting ability modifier" would/could be whatever spellcasting class she belonged to. If multiple classes apply, one should be chosen as the primary class and once the decision is made it'd be permanent.
Pretty much this. I left out the caster level requirement by mistake, which the rest of the text assumes I'd put in.
So, RAI, I can't have a fighter splash two levels in Souldrinker for the purposes of Happy Fun Times with a +N conductive longsword. Ah, well.
So, RAI, I can't have a fighter splash two levels in Souldrinker for the purposes of Happy Fun Times with a +N conductive longsword. Ah, well.
I never actually considered a non-caster taking the class (because of the presumption in my mind that you'd be dealing peacefully with bound or summoned daemons in some capacity to learn the PrC). However I suppose if you're part of a daemon cult and not a spellcaster, that's certainly a very cool option (albeit one that I never originally intended). The RAW don't preclude it, and on top of that I certainly don't actually have a problem with it (it just wasn't something I'd considered originally).
:)
Would an alchemist gain more extracts?
I could see the draw, certainly. Eat a soul, brew a potion. Eat a soul, brew a potion.
My first pass at this was an alchemist tiefling with a cacodaemon tumor familiar. Just liked the idea of eating souls for a little bit o' fast healing. Then I saw there was a prestige class and got all giggly and squee.
Is there an FAQ (or thread) rolling around about alchemists taking levels in P-classes with caster level or spell requirements, and that get "+1 to caster class" stuff? They aren't casters, but I can't fathom why we'd want to leave them out of this (in general, not just this specific p-class).
My current plan for souldrinker is to slap it on an Artificer (also not a real caster, and 3rd party material), so your question is relevant to my interests.