World turtle immobilised

Picture a world-carrying turtle. Four elephants on its shell, and a flat planet on their backs.  We want the turtle to be visible from the edge of the world, so have the turtle's length overlap the planet's by about an eighth at front and back.  That should give us a bit of flipper and a head to look at if we're peering down from the very edge, where the ocean turns into a waterfall that tumbles endlessly into space.  For the planet to have the same surface area as the Earth, that would make it about 19,000 Km or very roughly 12,000 miles across.  Big.

We want it to be smaller, because we're about to crash it into an inhabited world.  Let's make it a baby, only about the size of Japan.  That gives us 1,200 Km (750 Mi) of turtle, and makes the world on its back 900 Km (560 Mi) across.  Still big.  If we take a Loggerhead turtle as our template, it's a turtle the size of Ceres, the largest asteroid ever discovered.  It's not a dinosaur killer, it's more like the school bully that beats up dinosaur killers and steals their lunch money.  So instead it makes a semi-controlled landing and immediately dies because something much larger than it is has taken a bite out of its side.

(Sages of the day recorded that something whale-shaped blotted out constellations over several nights, other mystics point out that it had a triangular fin on its back.  Educated men know the world is round and can use parallax measurements to estimate the creature's size, but they quickly run out of beads on their abacus in the attempt.)

Even a controlled landing by a 1,200 Km turtle is going to mess up your planet.  First there's the noise, the loudest ever heard.  Birds are knocked out of the air by the pressure of the sound wave.  Then there's the hurricane of displaced air, tearing up forests and stripping fields.  The impact (no more than a kiss, really) sends millions of tons of soil and rocks billowing into the atmosphere, blotting out the sun.  All across your continent, dormant volcanoes erupt back into life, spewing magma and toxic gas.  Earthquakes shiver your cities apart.  The first of the great forests far enough from the crash site to have survived begins to burn.  Rocks the size of houses crash down, entire countries distant.

A third of your population dies in the first few hours.  Half that number again starve as crops fail and the landscape plunges into a winter that will last three years.  More are killed by bandits and wandering companies of looting deserters during the civil war that follows, or forced to flee into the wastes as refugees.  It takes a while for civilisation to re-emerge, establish a government and rule of law, and solve enough of its ongoing problems to think about sending an expedition to the site of the impact to see what the hell happened.  Two generations doesn't seem like an unreasonable length of time.

In the meantime, strange rumours have been filtering out of the East (which is arbitrarily where our turtle landed in relation to the new centres of government).  Never-ending salty rain.  An inland sea where none existed before.  Oddly symmetrical mountains on the horizons.  A shining globe above those mountains, half-hidden by a haze of distance.  Enormous, glowing spheres crashing across the landscape.  Strange, savage animals and plants.

It would be a whole new world, within reach of the recovering old world.  I think I could make a campaign out of that.