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?