I am sending my players into an ancient, abandoned Elven city. A 1000 years ago, the elves had firendly relations with the dwarves living nearby (the city is in a mountain valley), and the dwarves helped them carve their city into the mountainside. Well, now the city is nothing but lifeless stone, and all the trees that the elves planted have become petrified. So the environment is a large walled city, with lots of crumbling buildings and petrified trees giving it something of a maze-like feel. (The party, by the way, is level 7).
What I would like is some sort of monster to be stalking them amongst the ruins; a continuous threat that hampers their exploration of the city (but doesn't drive them away screaming) but that won't be just fought and dealt with all at once. I have overused undead at this point, so I need something of another type. Any suggestions from creative DMs would be extremely welcome!
Thanks,
R