Only when the stars are right can the Great Old Ones live once more.
Yes, they’re OCD like that. Everything has to be perfect. Sirius has to be aligned with Polaris but not with the rest of Ursula Minor. The Andromeda Group must be rotating at an angle not exceeding 67 degrees to the Milky Way. Jupiter, which isn’t even a star mind you, cannot cross the 45th parallel unless Neptune does it first. The Cygnus Nebula cannot have completely dissolved but the one in Sagittarius better not have kicked up yet. And you can forget it if it’s not at night that all of this is happening, not to mention that you have to be on the right side of the globe or none of this means anything. And it can’t even just be close. “What (gurglegurgle) Orion’s Belt only (gurgle) slipped an inch, not two? Why (slobber) did you wake me up? Lose 5d20 SAN.” Ugh.
And don’t get me started about when they can finally live again. We have to go out to their sunken cities in the middle of nowhere in a limited time frame or we have to wait for the next time the stars are right. How are we supposed to find their tiny islands in the Pacific Ocean anyway if they just suddenly pop up and sink back down without so much as a premonition or warning? And the geometry there is all wrong. We’re supposed to find a flat fifty foot radius surface to perform some last summoning rites and sacrifices. There’s no telling what flat will be on the island. Heck, the geography will probably be all wrong too. We’ll get dropped off there, and then the helicopter won’t be able to find us anymore because the island moved. It’s enough to drive anyone insane.
The worst part is probably their deathless dreaming. It projects and apparently drives artists nuts and causes weaker willed people to start more "traditional" cult activity, thus tipping off the Anti-Great-Old-Ones Force, the people with tactical nukes, and general busybodies with unusual compulsions. They come and investigate and then guess who’s going to sleep through their alarm because we’re knee high in arrests and deaths and nosy do-goods?
Do these unspeakable horrors want to rise from their unimaginable stasis of millennia or not?