It’s been noted before that OD&D is a post-apocalyptic setting. Much of what was amazing and wondrous has been lost. The greatest technologies are stuffed in chests in deep underground dungeons and whatnot.
I imagine most world-builders have a well-thought-out apocalypse. As I put together my campaign, I need to decide what my apocalypse was and how long ago it was. I’ll also pick some sort of disaster that isn’t too far off as well.
Campaign Apocalypse (Roll your poor neglected d12 once)
1- Invasion overtakes known world! (roll 1d6: 1- Mages on horseback, 2- giant ants, 3- elementals, 4- religious fanatics, 5- halflings, 6- human army)
2- Living Dead Return (If you don’t already have undead.)
3- World War! All nations are at war. It’s brutal. See WWII or A Game of Thrones.
4- Asteroid strike! Everything is mutated.
5- Extreme Weather (Ice Age, Desertification)
6- Geological disaster cracks continent in half!
8- Yuchen-Domma, Goddess of Despair, is freed to sing her dirge. Or other religious disaster.
9- Economic Disaster, courtesy the halflings. They manage to take over your nation or corner the market on healing potions.
10- Magic disappears (Clerical or M-U or both!) for a period of time.
11- Summon Slim Whitman — Mars Attacks! (you must be ready to transition campaign into space if necessary)
12- All children have level drain gaze attack. Which might also work in an RPG. Thanks very much everybody, tip your waitress!