KIC 8462852, aka Tabby's Star has gained attention as a possible proof of extraterrestrial life.
The following short story presents a potential description of the aliens' back story.
“I know we don’t have a lot of time,” Crulax said rubbing a tear from her eye. “But let’s recount once more our rationale for this measure.”
Spru nodded and mustered, despite it all, a simple smile across his chest. “It’s a message to all the intelligent life in our galaxy, even hundreds and thousands of years into the future." He glanced around the room, a darkened confusion of circuits. "Don’t follow in our footsteps.”
“But why do we conclude they can know what we became?” Priya asked. She already knew the answer. She herself was the one who had proposed the controversial measure.
Crulax closed her eye and took a deep breath. “Any intelligent life out there capable of observing what we leave here will have already learned that interstellar travel is unfeasible. You can launch intergenerational missions to the star next door, but the light barrier is insurmountable.”
“And that means…” Spru started.
“Everyone is alone,” Priya stated.
A moment of silence.
“How did we get to this juncture?” Crulax asked.
Spru looked to the ceiling and sighed. “Our divorce from nature, from our home world, which we depleted, led to ever grander living structures in orbit of our sun. Today five thousand or so modules, each one significantly larger than our home world, orbit at varying distances.”
“And what do neighboring stars conclude right now when they observe the light of our star?” Priya asked. Even she now smiled, realizing the import of the moment.
Her eye was still filled with tears, but Crulax was resolved and calm. “They conclude this system is full of regularly orbiting planets and comets. And that is why we three, the last remaining of our species, must do this thing.”
“Far too late,” Spru began, “we learned that living beings are a part of the planet that bore them. We waged wars against the viral diseases that stream through the universe. We eventually lost. Only now we know that it was our home world that gave us immunity.”
Another moment of silence.
Crulax herself now finally smiled, though tears still flowed from her eye. “We know we have to do this thing. We know that we three have just days left to live. We are going to send a transmission to all the empty orbiting modules to randomly change their speed.”
Priya actually laughed out loud. “We have the ability to send an SOS out to the entire galaxy. We have to believe, have to hope, that when intelligent beings see that the light of our sun drops by 20% at random levels, that they will draw what we know is the only rational conclusion.”
“Randomness is only produced by a mind,” Spru said.
“Planets and comets orbit regularly. We have to hope that the randomness is what intelligent beings will notice. It can only mean that a race of people have a left a warning beacon.”
“Again,” Priya blurted, “ What do they conclude?”
A final moment of silence.
Crulax nodded. “Working backwards. A random drop in light is the mark of a mind. And it is obviously produced by a mind with a message. And that message can only be that extra-planetary existence is doomed. And so, we three have agreed, before we die, we send the signal. The modules will now randomly change their speed, producing a light-signature we can only hope other systems will see. And if they read it right, they will choose to nurture the planet they inhabit, rather than deplete it and then abandon it.”
Spru smiled a final time. “I love you all. And I love them.”