Something like that happened in my game. I was to the southwest of the galaxy with four core systems and had expanded two remote systems to the northwest. These distant worlds were very near two fallen empires. If I compared the galaxy to a clock, the grumpier fallen empire was at 9 o’clock and the adorbs fallen empire was at 11 o’clock.
The weird extradimensional guys jumped in with a 50k stack. I was using hyperlanes. The system they jumped into had only one linked hyperlane, and that was through the grumpy fallen empires’ territory. There was no way I could reach their beachhead. Apparently they were fine using warp, though, and made a beeline towards the southernmost of my distant colonies. They quickly entered the system, and demolished the 2k fleet I had hanging around there. My main fleet was only about 20k so I knew I wouldn’t be hanging on to that planet, and probably not its neighbor either.
Right after I lost my piddly defense fleet, I contacted the grumps and gave them my world, free of charge. They didn’t do any due diligence, or maybe they didn’t care that the planet was about to be harvested by space ghosts, all zones from coast to coast. The fallen empire’s territory expanded wildly as it absorbed my erstwhile colony. And then it was gone, the colony was destroyed.
I hoped that would get the fallen empire to take care of the enemy for me, as there was no way I could do it on my own. Then the enemy fleet was spotted coming in towards my last remote planet. I sighed and contacted the other fallen empire, and deeded the colony over to them. Their territory expanded and returned to its original borders in the same fashion. I started building up my fleet, slowly, suffering from the massive loss in resources, but knowing I was too far to be next once again on the Harvesters’ menu. They had to go through a few other empires to get to my core planets.
Suddenly I got a message saying the grumpy fallen empire had destroyed the gate or wardrobe or whatever that the alien menace had slipped through, and the galaxy was saved. Then I got a message saying that the other fallen empire, the cute ones, had become very freaked out about the space ghosts and were stirring from their slumber. By this point, with the loss of those colonies, I was pretty far from the top empire in the galaxy. I wanted to throw in on the side of the cuties.
They didn’t want me though. I thought they didn’t like my empire’s stance on slavery, so I started making enormous social changes. My Golden Flock of Seagulls, led by Star Emperor BIRD PERSON, changed to an oligarchy ruled by clerics. Slavery was outlawed. They still didn’t want me fighting alongside them. I think it might be because earlier I gave them one of our population units to live in their menagerie planet so we’ll be preserved after our glorious avian race is inevitably wiped out; now they feel custodial toward us and don’t want to be the cause of the genocide they predicted. I was actually hoping to become their vassal. I’d love to actually finish a game of Stellaris instead of abandon it.