They're here.
Thormians rushed up and down the street, excitement brewing. On this day, calendar date 103-20-1029, the entire planet's hard work was about to pay off. The first prototype interstellar starship was finished, and they were about to leave their star system for the first time. But a mere ten minutes before launch, the event was interrupted by a transmission, a message encoded in the same way as the monolith in the middle of their civilization.
The ones we once considered Gods have arrived.
The monolith was scrawled with carvings of great asymmetrical beings, and the psionic waves it gave off could be 'heard' by any mind close enough to it. The simple message had made itself known to every living Thormian from the dawn of their species:
WE ARE THE OMNITHS, THE ONES-WHO-ARE-ALL.
THIS MONOLITH IS OUR GIFT TO YOU, THAT YOU MAY KNOW YOURSELVES.
ALL WE ASK IS THAT YOU PROSPER.
The message had been debated heavily throughout the ages. It had been the subject of religions, used as an excuse to cause strife or a reason to bring peace, and ultimately realized (with the advent of sufficiently advanced technology) to be what had uplifted the Thormian race from dumb animals. With the tech behind it nearly understood now, the agreement was practically unanimous: the monolith was granted by a benevolent alien species. Though why the gift was given, nobody knew.
But now they could ask.
As the first image of an Omnith showed up on every screen, they could see that the strange shapes of the carvings were no lie. There was a tinge of nervousness, no doubt, but the Omniths' gift extinguished those feelings before they could get too far. One could not help what they looked like, after all. Instead, they waited in anticipation for the first words from their progenitors.
"You have done well."
Cheers. Quadrupedal beings dancing as a worldwide cacophony sounded. No doubt their own works looked like toys compared to the great civilization the Omniths built, but to hear praise from the ones who brought them to this point was as a child hearing praise from a parent.
"We asked you to prosper, and you have done so. Ten great cities, at a level you all should be proud of."
The ten governors, all gathered for the starship launch, smiled at each other.
"Yes, it's wonderful. To borrow a phrase, it's a mighty fine crop this year."
Confusion.
"With the costs of construction and engineering, the limited abilities of colonists to adapt, and the time-scale that galactic travel works on, it is prohibitively expensive in terms of money, effort, and time to colonize any given world. Far more efficient to uplift an animal at random and let them spend the time building the infrastructure for us."
Horror.
"Worry not. You will not die. You will be transformed. You will lose those terrible symmetrical forms and become beautifully unbalanced."
A beam fired down. The waters of the world turned blood-red.
A/N: Monolith farming. Colonies can only have three cities, and can only harvest a small amount of spice. Home planets always have ten cities, able to harvest a large amount of spice, but all the naturally-spawning home planets only have cheap spice. So what if I wanted a large amount of other colors of spice, and only wanted to travel to one-third the amount of planets?
Plant a monolith. Wait for the uplifted species to reach the Space Stage, making the world their home planet (giving it a full ten cities). Then blast it with the Zealot Beam. Then paint the waters bright red in honor of the Omnith homeworld's crimson seas (optional).
Honestly there's a sheer existential horror in the idea of uplifting a civilization, granting sapience, and allowing them to reach the full pinnacle of a space-faring civilization, then wiping them out, for no other reason than because you were too lazy to build your own infrastructure for a colony.
But all of those races were symmetrical, so they obviously don't deserve to exist anyways.