For your major spider, why not roll up stats as you would a PC?
That's the idea, yes, hence the question.
C&T I-II-III have stats for unusual races, I'm sure there's a creature that is similar to a spider or intelligent insect type.
Actually, no, there isn't. The RoCo. VI has stats for all creatures in the C&T, thus for the major spider, but I'd assume it's for its
spider form and hardly for when it takes a human form. The C&T has the maazhat but they are insectoids (just a note: spiders are
not insects, but arachnids...) and the stats are for them in ant's form anyway. I thought about using the spider hengeyokai from the OC (since they're pretty much that, spider being able to take human form) but its statistics feel too low for a major spider, mayhap more appropriate for a greater spider in human form.
…talking about which, you should make your greater spiders actually such hengeyokai. Your PCs would be in a lot of fun (ok, mayhap the fun would only be for you
) when the greater spider they've captured suddenly takes human form. Especially since you want it (her?) to be a young one, thus with the human appearance of a young/child girl.
Since, as with most people, your PCs probably consider that "all spiders are the same" (which they are
not; for some reasons, they're also often called "insects" whilst being in a different class, being arachnids), you may have actually several kinds of greater spiders: the animal intelligent ones whose venom is harvested by the cultists, and the hengeyokai with light human intelligence. And do you know what? Your PCs actually caught one of the latter!
…who actually may help them locating and capturing the former one!