Slimes are janitors.
They were introduced long ago, slorping around in the sewers to help with the disposal of waste. Gelatinous cubes were engineered to perfectly fit the long tunnels and make several passes in the same muckish water week after week. A month in, the waters looked almost pleasant.
Then there were black slimes, introduced as plumbers. Simply slip a packet of dried black slime into your toilet water and watch the messy contents drain themselves. Where did the black slime go? I dunno, probably the sewer. Did you want to buy another one or not?
Then the trash. Who wants to deal with trash? No one, so naturally the gray oozes are conscripted into service, living at the bottom of garbage cans. Soon enough one obviously hysterical woman screams that a gray ooze that thanked her for discarding her leftover waybread into its bin. Authorities are contacted, but nothing comes of it.
“My good people, oozes are virtually harmless.”
A strange tank shows up in town, its contents hidden behind a ratty wool blanket and is placed in the town square. Is that thing full of liquid? One child starts a rumor about a BRAIN being in its thick waters…
Perhaps it’s simply another slime to better society. Best not ask…
Next, food waste and manure. Let’s see, who’s left?… Ah, the ochre jelly! Yes! They can’t dissolve bone or metal, but surely the softer disposable contents would do them no harm. A true win-win.
A few families even take the slimes as house pets. Now the folks who house and cuddle their pet rats and snakes finally have someone to look down on. “What if they attack? What if they eat you in the night?” Nonsense. Anyone can out-walk an ooze…
Small display cases and larger glass containers start to sell really well, as people are fascinated with their new gloopy neighbors.
Janitors are replaced. Sewer workers are laid off. Garbage boys become slime handlers.
The city cheers. The mayor is praised. The wizards quickly abandon their towers, foreseeing what happens next.
The ratty wool blanket covering the strange tank falls. There was a brain in there. It has intelligence.
The baton-less conductor brain orchestrates the ochres and cubes, jellies and slimes in an uprising. Waves form of multicolored masses. Truly a stunning display of unity for slimekind.
The tsunami swamps the city.
There is no out-walking. Or out-running.
The slimes melt everything.
The self-evacuated wizards warn the nearby towns.
“What’s that? Gelatinous cube, you say? Best be rid of it. Don’t let it into the sewer, for then the problem compounds. Best take out the trash to the bin yourself.”
-Perhaps the adventurers are hired as “pest-control” in the sewers to “keep the numbers low.” Perfectly low-level.
-Slimes are being sold as pets. “Would you like one? This one can clean the mud off your cloak and shine your shoes!”
-The brain contacts the adventurers to get them on its side, promising all an ooze can offer.
-The adventurers arrive and hear that all the city’s wizards have left. “Quite a hurry they made too.”
-The adventurers encounter the city as a ruined one. The slimes rule here, with lots of the food now gone. The brain is dying for some entertainment.
-What other animal, monster, or spell could be introduced to a city for it’s, erm, betterment? Solutions that become problematic are often dynamic situations for players to step into.