
dragonhunterq |

They stack.
Monster Advancement Rules wrote:
Step 2:"If the creature possesses class features (such as spellcasting or sneak attack) for the class that is being added, these abilities stack."

Klorox |

Dominick Nate wrote:
Cool, thanks. However the Drider stat block states "A drider casts spells as a 6th-level cleric, sorcerer, or wizard, but does not gain any other class abilities." So do the levels stack for all purposes or only for spell strength?
that's for a "raw" drider. A PC drider with actual spellcaster levels to boot would have class abilities as per his class levels, and spell casting stacking with what he had as a drider.

dragonhunterq |

Amaranthine Witch wrote:
If a drider with cleric casting gained 2 levels of cleric, he would have the spellcasting of an 8th lvl cleric and the class abilites of a 2nd lvl cleric.
This witch gets it.
This applies whether PC or NPC. There is no material difference as far as the rules are concerned - beyond stats and wealth anyway.