Congratulations! Your players have become the villains of the campaign. What should you expect when this happens?
Well, summoning an elder god is an extremely stupid foolish idiot thing to do. An Elder God cannot be reasoned with or controlled by a pathetic mortal and attempting to get its attention will likely get a dozen square miles flattened like God swatting a flea.
But if you’re arrogant or greedy or shortsighted enough to want to do it anyways, then you’ve got a lot of work to do. At the very least, you’re going to need a complete copy of the necronomicon, which will be near-impossible to find and definitely impossible to retrieve without committing some heinous crimes. Plus you’ll need some ritual artifacts from cyclopean remnants deep beneath the sea or under the ice in the antarctic. And to get all those, you need money, power, and connections so I hope you like dealing with the Mob. Plus your body will need to be altered to survive channeling that much arcane power, so I hope you like mutating into something that makes Wilbur Whately look like Adonis.
And naturally while you’re doing all this, a group of random shmoes will stumble onto your conspiracy and band together as a group of Investigators to try and stop you beginning an apocalypse. There’ll be some back and forth as you send minions to deal with them, trap them, race them, etc., but they almost certainly will be there right as you are culminating your great summoning ritual. Then it’s all up to the dice: either you win, summon an Elder God, and get everything in the zip code including yourself killed for annoying it; or you lose, and an investigator puts a .44 through your soft cartilaginous skull.
Pretty much
Naaaaaaah.