What Happens When Cthulhu Is Released

The word "lovecraftian" doesn't mean tentacles and cultists and insanity.  It means that mankind is utterly inconsequential.  The universe isn't just weird and hostile, it is incomprehensible and unaffectable.

Humanity is worthless, our souls are worthless.  All of our religions put humanity on a pedestal--we can imagine anything, we can advance until we can do anything, we can eventually understand everything, we were created special, this world was created specially for us. . . and these are part of a human-centric worldview.  The lovecraftian worldview is that humanity is incidental and meaningless.  Comparing humanity to elder gods is not like comparing ants to humans, it is comparing colonies trapped inside a petri dish to duelling, four-dimensional suns.

The elder gods don't want anything from us, because we don't have anything they want, or could ever want.  We cannot threaten them or affect them in any way, because we are scum on the inside of a petri dish.  We could, conceivably, draw the slightest iota of their attention, but that would be it.

Cthulhu is trapped in the dungeon-crust of Centerra's planet.  Cthulhu is not an elder god, but it is several order of magnitude more powerful than anything else on the planet.  He has, like, a million HP and immunity to magic.  This is what happens if you wake him up.

What Happens When Cthulhu Is Released

Every sentient creature on the planet must save or go permanently insane as the Cthulhu's supersentience rips through their own, like a cruise ship powering through a narrow canal.

Over the next 24 hours, Cthulhu devours the sun, in order to gain enough energy for a long flight home.  Then he flies home, never to be seen again.  A wish spell might delay him, briefly.  Then the planet has no sun.  It's just a rock hurtling through space.  The surface temperature plummets, all surface water freezes, and 99.99% of all life on the surface dies.

Suddenly the underdark is looking pretty good.

The campaign changes to survival.  The PCs must travel through the interior of the planet's crust (which is all dungeons and subterranean oceans and literal hells) to get to the interior of the hollow planet, where the interior sun provides a modicum of ghastly light and warmth.  If the PCs want their species to survive, they had better bring along enough humans/hobbits/whatevers to form a breeding colony on the inside of the planet.

Over time, the helpless-insane will be killed and eaten by the murderous-insane, who will form roving packs of cannibals.  The psychic weave around the planet is completely changed when the cold tonnage of Cthulhu's brain ripped through it.  Psychic powers begin to emerge, and the people who were best poised to take advantage of this change (the Cthulhu cultists, who were not so much crazy as they were desperate) come out on top, and many of them become powerful, immortal god-kings.

Everything freezes.  There is no food.

The campaign changes to building a settlement on the interior of the planet.  It's like zombie survival, except with morlocks, demons, and neothelids (worms of the third madness).  Or maybe a bit like +Gus L's Underdark, 

If the players get ahold of a morlock drill city, they'll be able to make journeys to the frozen wasteland that is the surface, and search for valuable stuff in the ice-clad cities.  They'll skate across frozen oceans, their ice-sleds pulled by kites.  They'll move from volcano to volcano, the only points of life on a dead surface.  Each volcano is a desperate stronghold, a feeble source of food in a dead world.

If this campaign has an end goal--and it doesn't have to--it is to slightly change the trajectory of the planet.

Hurtling through space, the dead planet is doomed to a wretched infinity of cold vastness.  Humanity exists, if it exists at all, as a pathetic race living in the most meager of conditions, in those tiny niches of the planet that are still capable of sustaining their hot-blooded, gluttonous lives.  And that's where they'll stay while the planet inevitably cools and dies.

But with a small change, the planet could be redirected.  It could be recaptured by another star, and it could eventually orbit that star and flourish once more.  It will require a volcanic eruption of inconceivable energy, precise mass, and precise trajectory.  The planet needs to jettison a chunk of itself in order to life.  And not just amputate it--it needs to fling those oceans and continents as hard as it can.

People (and PCs) who are Nikal can survive being frozen, and might be revived when the surface thaws, far into the future.

And the rewards of this ridiculous maneuver will not be felt by the party.  Nor by their children or their grandchildren.  Nor their great, great, great grandchildren.  But eventually, the planet will be inhabitable once again, and the suffering will not have been infinite, and there will be green fields again, and children will run on them and laugh and speak in alien tongues, long after you've died.

And that is victory, perhaps, in a lovecraftian sense.