There's a sub-sub-genre of RPG scenarios called "The Last X Adventure," intended to wrap up a campaign and/or blow up a setting. In honor of Terry Pratchett's memory, I present . . . The Last Discworld Adventure.
The well-dressed merchant snarled as the street vendor sidled up to him. "I don't want to hear your latest get-rich-quick scheme, Dibbler. I lost too much on them already."
"One word, guv'nor. Octoleum."
The dwarves of Klatchistan have delved greedily and deep. After finding gold and more gold, they found something new. A thick amber liquid, warm to the touch, which came out of the hole as if under strong pressure. They pumped it out to clear the way for more gold-digging and made a discovery. Pumps coated with the liquid would work without dwarves pumping them. When applied to elevator cables, those too would work without anyone's effort. The liquid would evaporate, but there was plenty more to apply.
A wizard brought in to analyze the substance named it "octoleum" and ran screaming. But the dwarves noticed no ill effects from using it. Experiments showed it could enable carts to move without horses and unmagical carpets to fly. Klatchians eagerly spent their gold for kegs of octoleum to do their work for them.
In game mechanics, octoleum will turn any TL3-4 machine into its TL7 equivalent. A cart performs as an automobile, a carpet as a Cessna, etc. An ounce of octoleum roughly equals a gallon of jet fuel, gasoline, diesel, or whatever fuel is needed. Steering must be supplied by a driver but the octoleum will start and stop as the driver/owner commands. A container of octoleum must be permanently fastened to the machine. When it runs empty the machine reverts to its original nature (carpet pilots should check the level often).
The players find out about the discovery through rumors or the arrival of an octoleum salesman in a horseless carriage. It is initially be used by the very rich and industrial efforts needing concentrated power. Shortly after a Klatchian dwarf with far too much gold will arrive and disrupt the local economy by buying up luxury goods. (Another impact could be additional funding for the dwarf terrorists from Raising Steam, if your players haven't had enough of that sort of thing from the news).
People all over will want to dig for octoleum themselves, as there's far more demand than Klatch can meet and transportation expenses are driving up the price. Arguments over who owns the rights to drill where can produce fights needing PCs to settle them, or inventive PCs could be hired to make a hole.
CMOT Dibber organizes an off-shore drilling effort off the coast of Ankh-Morpork, just far enough out to see to avoid legal complications. The Patrician will offer to hire the PCs as observers on the scene to ensure the city's interests are protected. In Lancre a few Ogg grandsons see a chance to make their fortune. If they do, they'll be paying for Nanny to buy an impressive mansion somewhere. Possibly in Ankh-Morpork, next door to a very old and dignified vampire family, if the GM wants to transport the Beverley Hillbillies to Discworld.
The impacts will be greater than Nanny and sixty descendants filling up a twenty-room mansion. Carts will fill the streets. Special speed-carriages will start racing each other down the avenues as the Watch try to catch them on foot. Suburbs will displace turnip farms. Pipelines will draw protests. Not to mention what happens when someone spills this stuff . . .
Actually, spills aren't dangerous if no one is watching . . . or if no one has any ideas about what should happen. Octoleum in the tank of a mill, cart, or carpet will obey the owner or driver. If it's lying loose anyone's suggestions will be obeyed. Which leads to little boys sabotaging tanks so they can watch cobblestones fling themselves about.
This should all be background as the PCs go through the regular campaign, unless they want to be part of the boom.
Unseen University is investing massive effort into understanding the stuff. All understanding of magic thinks a keg of the stuff should immediately induce an incursion from the Dungeon Dimensions. The deep reservoirs should have been swarming with the creatures. The High Energy Magic Lab sets up new buildings to tackle the problem in (because large amounts of octoleum can explode if someone wants it to). Wizard PCs will be recruited as part of the research team. Non-wizards could be lab assistants ("Wait until I'm behind the stone wall, then push this button.").
The greatest discovery is that octoleum contains living things, apparently the young of the Dungeon Dimension creatures. The fluid they swim is rotted creatures or possible worse products of the DD. And yet . . . no incursions. Ponder Stibbons concludes that the contents are actually repellent to the DD creatures, making octoleum perfectly safe to use.
With UU's admission that it's acceptable for general use even more effort goes into drilling for octoleum. Production increases, new uses are found, and an industrial revolution is spreading across the world.
Then the earthquakes start. Tiny ones at first, barely noticeable. Geological hobbyists are the only ones who see the trend. One will buttonhole the PCs to proclaim his findings. No one believes them until the quakes become large enough to shake things.
Everyone who's been displaced from their old positions of power and wealth by the octoleum boom promptly blames the quakes on the liquid. Everyone benefiting declares it a coincidence. The wizards have no idea, and request funding to investigate. PCs will be wanted to participate.
The quakes are distributed randomly, usually happening about a quarter of the way in from the Rim. If researchers can check for quakes under the oceans they'll find the distribution is quite even around the circle.
As the quakes keep worsening, someone decides to send the PCs to look at the underside of the Disc. One glance at the elephants reveals the problem. They're bleeding, as if sandpaper has been rubbed over their backs.
Octoleum was the lubricant that let the Disc spin atop the elephants. Now that its diminished, friction is building up. Each time the rock of the Disc hits an elephant the surface feels an earthquake . . . and an elephant feels pain.
The news is greeted with panic and disbelief. Some make an effort to stop all drilling, but there are octoleum wells all over the Discworld. Some are closed by mobs or armies but the fluid already pumped out has been used up.
Frantic plans are offered to solve the problem. Some hope the reservoirs would refill on their own if the drilling stops. Others try turning the Fifth Elephant deposits into synthetic lubricant to be pumped into the empty drill holes. The most desperate propose invading the Dungeon Dimensions to obtain replacement fluid for the reservoirs (and are greeted with "No Blood for Octoleum" signs).
The PCs will undoubtedly come up with an even more bizarre plan to save the Discworld. It will fail.
True terror will begin when a nasty quake breaks off a piece of the Rim and it falls into space, lost forever.
Captain Carrot then has a series of tasks for the PCs. First they need to find drillers and have them make a circle of holes around Ankh-Morpork and the nearest farms. Next they need to accumulate all the available octoleum, not just pumping more from the wells near Ankh-Morpork but importing kegs by bribery, theft, or force. If necessary he'll organize troops for the PCs to command. He also sets the Bricklayers and Glassblowers to building a glass dome over the city to just inside the drill holes. Magical PCs will be tasked to strengthen the dome so it will survive quakes.
Eventually one of the elephants can't take it any more and collapses, falling off the back of the Great A'Tuin. The rest follow in minutes. The Disc crashes down on the astroturtle's back and shatters.
Ankh-Morpork holds together. The bedrock splits along the line of the drill-holes, leaving the glass dome intact. The people inside have air to breath as they watch the rest of the world destroyed.
Captain Carrot, no longer of the Watch but Captain of the city-spaceship, sets out to find a new world for the city to land on. Perhaps another astroturtle is out there, or one of the fabled round worlds? Or Ankh-Morporkh may sail between the stars until it runs out of octoleum (cf. Blish's Cities in Space). The PCs will become Captain Carrot's away team.