Hm, better idea. Regular spiders would be too heavy to walk on the ceilings, but cave spiders can. Swap out the long poison effect for a short one, then let them camp out in groups on high, dark roofs. When you walk under them, they slowly, silently descend. If you catch them, they screech and lunge at you...
As someone who finds spiders, in the most manly way possible, 'icky' this idea is terrifying.
Any of you ever use the Divine RPG mod? I thought about trying it out but when I asked about why they made it so difficult they said vanilla was too easy.

I've used it a couple of times, it's fun, don't remember it being horribly difficult though.
What's the status on the FTB server? Is it on ultimate yet?