I'm planning on soon running an adventure that's a somewhat more traditional dungeon crawl--going from one room to the next, traps, etc. The PCs will be exploring a long-lost vault/temple (of good clerics) buried deep under a mountain. No one has entered this temple for hundreds and hundreds of years, until the PCs finally have the key. I'm trying to think of some good monsters/creatures for them to encounter. PCs are 8th level, but I can scale things any which was as needed.
Difficulty: I want to avoid using undead (this place is supposed to seem less desecrated than future adventure locales). I'd also like back away from constructs and elementals, as my PCs are crit and sneak-attack focused. I'm also not really a fan of "oh, this monster has tunneled into the temple", because that would break the whole "this has been sealed forever" feel I'm looking for.
Any ideas? Either monsters from the myriad of books that have been published, or just a basic concept that I can stat out. What would be a fun creature to encounter and fight in a long lost temple vault?