Warning – this whole post is a spoiler.
In the 3rd Imperium setting of Traveller there are a species called the Ancients who did stuff and a boss dude called “Grandfather” more or less detailed here:
Anyway in my quest to include every existing sci fi trope in the Spinward Marches…
Grandfather and the actual Ancients are D&D mind flayers / Cthulhu Starspawn type creatures – the Droyne are a servitor species.
They developed immortality requiring some kind of vampiric process.
They were big on genetic engineering and mass gladiator battles.
They picked up humans and modified them, sometimes just for fun, sometimes for practical reasons so they could work more easily on a particular world with droyne as slave drivers but they also used them for war-sized gladiator type battles e.g. Waterloo or Kursk sized, including Mecha type battles and battles against giant robots – Vland still has a cultural memory of these. The gladiator humans were generally left unmodified to make the battles even.
The Ancients liked to bet on the outcomes of the battles.
Sometimes Ancients would control the battles directly using psionic powers to give orders to the officers on the ground like a computer game with real blood.
Anyway apart from all that the Ancients built all kinds of high tech wonders – artificial planets, ring worlds etc blah blah.
They also built a robotic fleet with an AI which – you guessed it – went rogue and developed a version of the Prime Directive: that it was wrong for a space-faring species to interfere with a non space faring species. After the Ancients refused to obey the Fleet’s order to desist (and as despite all their psionic powers they couldn’t regain control of their robotic forces) the Fleet attacked and destroyed all the Ancient’s industrial worlds, anything capable of building ships and anything that could become capable of building ships.
Only Grandfather and 420 Ancients survived this destruction most of them on research stations on worlds with little other infrastructure.
Fleet moved out into the universe looking for other breakers of its Prime Directive but not before leaving behind TL18 stealth ships containing hunter killer terminator robots to hunt down the last remaining Ancients.
Grandfather escaped into a pocket universe while the others were hunted down – not all were however, many of them found a way to hide. The terminators, stealth ships and Ancients are still there in the OTU hidden.
Consequences in Game
– hidden TL18 stealth ships in system with unaccounted for Ancients, literally hidden just sitting on a moon somewhere powered down for thousands of years waiting for a scent
– Terminators, some on the stealth ships, some on planets. The ones that got their target designed to self-destruct but those who never found their target just power down and wake up every few centuries to sniff. Invisible in stealth armor one could be sitting on the roof of an Ancients site powered down while Imperial archaeologists work below.
Terminator – botched self-destruct, mangled Terminator somewhere
– Surviving Ancients who hid themselves in various ways to evade the Terminators: cybernetic Dalek form, bio-engineered form, stasis, sealed inside an impregnable tomb etc
– Grandfather, if players find his pocket universe he tells them various lies to get them to check out systems where there may be a surviving Ancients and/or tries to get them to find and destroy Terminators for him. Say c.120 of the surviving 420 Ancients were killed by Terminators and he’s rescued c. 100 over the years and he’s still trying to gather in the remaining 200 or so.
– wandering Ancients – possibly transformed to hide scent from terminators
– misfiring Terminators – an Ancient created an item to act as a decoy which gives off Ancient’s scent, players find it
As mentioned Vland retained a cultural memory of their role in the Ancients gladiatorial games particularly the Vilani gladiators having to fight giant robots. This passed down through festivals and semi-religious symbolism but after the development of grav tech spawned an entertainment involving giant Mechs with teams of players inside controlling them. This is now an interstellar sport with a league in many sub-sectors.