The Tiamat-Ymir Principle.

Big creatures are vital to mythologies everywhere. They are the movers and shakers of the world, sometimes literally. They can be gods, or guardians, or monsters in need of slaying by a hero going through their standard-issue 17-point plan for greatness. And while their importance might often end with their deaths, their being dead can sometimes be of equal or greater significance to the narrative.

Sometimes the death of a Big Thing leads to the birth of much greater things. Even something as big as a world.

Mythological Context (AKA Feel-Bad Stories)

When the Babylonian goddess Tiamat discovered that her husband Apsu had been murdered by their children, the first generation of gods, she was furious. She took on the shape of a terrible sea serpent (or dragon, in her more pop-culturey depictions) and made a war of vengeance on her treacherous sons and daughters. She conjured the first dragons, and other monsters, as tools of her will. And when her son the storm god Marduk killed her, she was carved up into two halves and then mutilated.

Her eyes, still bitterly weeping even after death, were made into the sources of the Tigris and Euphrates rivers. Her ribs became the vault of the sky, her lower body the earth. Her tail was flung into the void to form the Milky Way. She was big enough to be made into the world that we live on, and in that way the Mother of Monsters is humanity's mother as well.

When the North Germanic giant Ymir was born from the thawing water that existed between the frozen realm of Niflheim and its fiery opposite Muspelheim, he didn't do much more than sit around sweating and drinking cow's milk straight from the tap. But from his armpit sweat and chafing thighs were born the jotun people, and from the salt lick that the cow Auðumbla tongued placidly was born Búri, ancestor of all the gods.

Eventually Odin and his brothers--all of them distant descendants of Ymir thanks to their jotun mother Bestla--came to slay him. Not because he had done anything in particular against them, though the gods did seem to think the giants were evil, even back then. They killed him because they wanted to become the rulers of earth and sky- things which did not yet exist in the yawning emptiness of Ginnungagap. So they fashioned the earth out of his flesh, mountains from his bones, trees from hair, the seas from his blood, the sky from his skullcap, storm clouds from his brains, and the entirety of the human realm of Midgard from his eyebrows.

Of course there are thousands more beings that die like this. Some of them are more benign and peaceful. I don't know of any existing term for it in proper mythography, but apparently Jungian theory calls it the Cosmic Man.

So What?

When Big Things die, they turn into bigger things. Like normal flesh-and-blood creatures they don't just vanish into the ether. They have bodies with physical pieces and parts, and just as we decay and get reused by nature, these parts can be separated and made into something else.

So what if this Tiamat-Ymir Principle™ applied to other things?

What if every significant beastie in a universe was able to turn into something upon death? And what if this process could be directed and controlled by the smaller creatures that kill them? Not just in the sense of cutting off a dragon's horns for weapons or scales for armor, but shaping an entire fortress or range of hills out of its body.

What if adventurers were less like thieving murder-hobos, and more like landscapers (but also still thieving murder-hobos)?

You get a setting where the classic pass-time of going out and killing big things has immediate, physical consequences in the world that even the least attentive party can see. The landscape can be transformed by accident or by deliberate act, either as a plot twist or as part of a plan made by any players clever enough to kill a titan.

What To Do With This

At this point I'm just throwing out whatever sounds nifty. It's not a unique idea at all obviously, but I never saw it called out and named before. Feel free to appropriate the idea however you see fit.

Landscaping- The world has always been home to colossal things and the little pests that murder them. It's a way of life. Hunt something, kill it, fashion a civilization out of its insides and then eat the rest. Monster Hunter, Salt in Wounds, etc.

But here, every landmark has a bloody, meaty history. That island in a lake was a wyrm that hit the ground so hard it formed a crater. Those mountains are actually a graveyard of stone giants being riddled with tunnels by dwarves like termites in so much rotting wood. This city was built with the fingerbones of a Hekatoncheires.

The world is defined by the monsters that live and die on it, and in attempting to master their environment the mortal species take a more hands-on approach.

The consequences of Big Thing death can be just as destructive as they are creative, of course.

Killing the monster harassing the town might make the road to the town impassable, or snarl the area in marshes, isolating the settlement from trade and travel and killing it more slowly. A wounded or naturally old being might be herded away from populated lands so it doesn't completely throw the region's topography into whack. Sending a titan running amok into enemy lands could inadvertently give them building materials for a new castle.

Maps could be made utterly useless every few years thanks to a brand new chasm or forest that definitely wasn't there before. Travel is never entirely secure. The world is living, breathing, and growing with every death, stacking up high on the literal bones of eons.

Terraforming- Space is a lot of empty, well, space. There aren't a lot of habitable worlds out there for most humanoids. But the void is home to giant things, living or dead. Think of the astral god-isles from Planescape. Most of these petrified deity remains probably occur from gods naturally losing followers and slowly decaying, but some must have been made suddenly, and by violence. Setting deicide aside for a moment, think of all the titanic alien beings that exist out in the gulfs of space, eldritch or otherwise.

If a sufficiently advanced civilization wanted to expand beyond its homeworld, but it could find no suitable planets to claim as their own, why not just make a new one?

Our "heroes" would be science-fantasy terraformers, tasked with tracking down a space Big Thing, killing it, bringing its corpse to a desired location, and then shaping its blood and bones into something that resembles home. If the creature had parasites, gestating young, or just a whole lot of gut fauna, perhaps this new world will offer unexpected competition or neighbors. This could be a process that takes centuries, or sufficiently powerful demigods could take care of it in a campaign or two.

And if the universe doesn't have the right kind of teleportation or flight magic, they're going to have to make a spaceship out of someone first.

Homegrown Gods- Paradise in all its forms has some overarching similarities, but just as many qualities are tailored to the people of a historical time and place. In many fantasy settings the gods either populate ontologically distinct planes after coming into existence, or they helped create them when they splortched this new universe out of cosmic goo.

Why not take this a step farther? In settings where gods are created by the hopes and prayers of mortals, and gods create their own planes, then are mortals not indirectly creating those heavens and hells for themselves? What if they cut out the middleman, so to speak?

Found a religion. It can be about anything, as long as it has a god or godlike being, and that being can be imagined in a body. Gain followers. Feed this blind, mindless godhead the thoughts and souls of thousands of like-minded people until it is a polished mirror reflecting back at them. Nurture your nascent deity, foster its growth and development. Teach it with ritual and scripture. Reward it with sacrifices like treats to a good child. Describe what the afterlife is like in your faith. Make your god strong enough to embody it and protect it.

And then kill it.

Butcher your god. Take its bones and craft a firmament. Shape the new land with its meat. Fill the air with its dying breath. Use its skin to line the boundaries of this new realm.

With your pandeicide complete, you and your followers may now enjoy an eternity or however many eons or kalpas your afterlife is meant to last, all to yourselves. Belief will continue to bleed into the landscape overtime, letting your infant plane grow.

Just don't let it slip to your living followers that god is dead and you killed him. Some philosopher might latch on and make a big deal of it.

Hunted- They're after you. The fiendish little upstarts. They've stopped playing with fire and digging holes in the dirt long enough to decide to kill you. Their champions are on the way, with magic potions and bright iron. Perhaps some of your kin assisted them. But it doesn't matter. All that matters is that they want to inflict that final and absolute shame upon you. They won't just kill you. They will take you apart and desecrate you. You will be devoured by lowly little things that are not even close enough to you for it to count as cannibalism.

There's no true escape. Chaos is vast, even infinite, but the hunter is tireless. They will always be right behind you. You can fight back, but they are great- greater than you, even. Your hubris would kill you before their swords did.

The best you can do is deny them their prize. Fight tooth and nail, and when those teeth and nails fall off, devour them. Allow nothing to fall into the hands of the enemy, for they will grow mightier for it. Live a perpetual fighting retreat. Leave a bloody trail across the length of the abyss.

They want a world. You will give them a battlefield.