I just read Tad's Seventy Spires world idea, and it reminded me of an idea I'd had for a D&D campaign, ages ago. All things considered, it might actually work better as a MultiVerser adventure.
I called it the Towers of Tal'Mineas.
Tal'Mineas began his life as simply Min, a lowly Imp, one of the lowest of the low on the least layers of the Nine Hells. In all the vast Multiverse, though, his lucky day was, well, one Hell of a lucky day.
No one is quite sure why or how, but he happened to be in the right place at the right time, when the Negative Energy Plane, the Abyss, and the Nine Hells all happened to cross in a planar sort of way. Most creatures would hardly notice this, but Min was a servant of Orcus, and had rather a lot of Negative Energy in him to begin with. So when the three Planes crossed, he noticed. He saw the convergence.
And he dove in. Call it impulse, or suicidal tendency, but he did. And he was rewarded for it. All that energy flowing into him energized him. Invigorated him. He wanted more, and he got it. All he could ever need. It turned him from lowly old Min, to Tal'Mineas (as, in my world, Devil names get longer and slightly more complex the higher one's rank - the prefix 'Tal' indicates a Lord of the Pit, and both names capitalized indicates one of the Lords of the Nine Hells). But as quickly as it came, he felt his nigh-godlike power slipping away.
So he did something about it.
Enter our adventurers. A few days ago, this weird black rock popped up out of the ground. Nobody thought much of it at the time. It looked like a volcanic rock, and maybe it had come from somewhere, but there are no volcanos anywhere near here, so nobody worried. Until the grass started dying. And then the sheep. And then Old Farmer Gildenstern. Yeah, people noticed when the old man died.
Basically, the Towers are like nails Tal'Mineas is using to hold the planes in place, so his power won't drain away when the Convergence ends. The problem is that the Towers you see aren't the real Towers, and they aren't really there. Walking right into one puts you on the other side of it, because the Towers aren't really Towers, they're more like one singular Tower that exists in many places at once. You can't destroy that Tower by attempting to destroy the many Towers, though it is possible - though difficult - to push a Tower back out.
You have to find the actual, original location of the Tower (on the site of the Convergence on the first of the nine hells) and destroy it in order to stop the chaos they're wreaking everywhere else. Basically they radiate one of the energies of the three planes Tal'Mineas draws power from.
A Negative Plane tower kills everything around it in an ever-increasing radius, and raises the creatures it kills as undead.
An Abyssal tower radiates chaos - things randomly turn to gold, or water, or butterflies appear from nowhere, or Genghis Khan arrives and declares victory. Things aren't necessarily bad near an Abyssal tower, just weird.
A Hellish tower radiates evil and aggression. People in the vicinity of one will become increasingly short-tempered and violent, to the point of being almost randomly homicidal. (The influence of the Abyss on Tal'Mineas himself adds Chaos to what would be a Lawful Evil - and thus colder, less murderous - effect.)
These effects start small and get progressively worse as the towers get higher (Tal'Mineas driving the nails ever deeper).
