The Old One Empire gets jiggy

Era beginning/end

These guys make you want to scream no, but the fact that them not being around would've butterfly affected humanity from existence makes you whisper yes. Or at least that's how it goes for me, someone who is possibly too biased in favour of humanity not dying. Then again these guys totally would've wiped out all life on Earth before humanity had even come to be if only they were able to access it.

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So, the Old Ones already existed for a long time in an ape-like state, not really being driven to innovate as they were extremely successful in their environment off of biology alone, but then some aliens showed up to mine their minerals, which kind of left a lot of the land that the Old Ones lived on infertile and they didn't like that. Utilizing a combination of them being naturally terrifying creatures, their communication skills to gather large forces from across their planet and their abilities to replenish their numbers very quickly, they were able to get these aliens to abandon their mining operations on the planet. Due to these aliens realising that these Old Ones were extremely hostile, that they were intelligent and the fact that these don't mix very well, said aliens decided to make the surface of the planet uninhabitable, resulting in the surface air being so acidic that the aliens themselves couldn't restart their mining operations as their suits would be burnt through very quickly.

Meanwhile the Old Ones were living underground for the next few centuries, building up massive cities using technology they'd acquired from their war with the aliens. Due to their natural lack of inventiveness most of their use of the technology was simply by either copying how the aliens had done things with this tech or by just doing what they previously did but with new tech, which made improvements very slow.

Eventually, however, the air on the surface reached more acceptable pH levels due to new plants appearing that absorbed the acids and were largely immune to them (this was through natural selection far more than to mutation, plants with such characteristics had already existed) . Thanks to this, the Old Ones were able to emerge once more to the surface and were not threatened by those aliens again, because said aliens were busy elsewhere trying to form an "if it's intelligent it's holy" cult with some other alien races and it wasn't going too smoothly. The Old Ones built up even more here with their acquired tech and got further advancements by raiding many of the ships that crashed onto their planet (as the moon of their world had been turned into a inter-species trade hub within the intelligence cult, but their planet was being left alone because, as much as Old Ones could survive it, none of the alien cult members could and their attempts at bio-domes had all failed).

Eventually, the Old Ones got around to a unified society with their most ancient, as Old Ones cannot die of old age, as their leaders - with many of them remembering and wanting revenge for the actions of the aliens.

Next they got spacey and the alien cult was like "oh my gods yes more space buddies real sorry about what our great x 45 grandparents did to you but I'm sure we can move past that right guys? Hey, here's a load of ships we're donating to you for joining us so you guys can get around easily within our unified space cult empire... wait, you are joining us right?"

The Old Ones were not joining them. The Old Ones were not joining them at all. The best part is that these aliens sent them blueprints for how to make themselves space suits and whatnot and so really did facilitate being attacked by never-ending hordes of Old Ones.

First, that moon base fell and the Old Ones acquired all of the technology, sent knowledge of it to their buddies back on the planet (fun fact: Old Ones have brilliant memories. Yeah they're a pretty overpowered species) and tonnes of stockpiled resources were quickly put to use to build up a massive space force that, while still outnumbered by the alien cult members, did have the advantage in other areas.

You see, as much as the Old Ones weren't really that good for strategy, mostly just copying what their enemies did, they could always just land a couple of attack ships with Old Ones that were genetically modified to survive in the environments of other planets on the planets they were attacking and within a few months that attack force would've grown in number enough in the wilderness and still known its cause well enough to go and conquer the planet once it had the chance. In other words they only needed to get a single genetically modified spark onto each enemy planet for the entire thing to go up in flames if only given enough time. Eradicating these Old One hordes on planets was also insanely difficult due to the fact that even 1 getting away would be enough for tens of thousands to be roaming the wilderness within a few months and that these wild Old Ones were extremely dangerous.

Due to this, they were able to quickly grow and internally diversify as a space empire into the territory of the this space cult and ultimately reigned victorious.

Now, what to do with all these alien space cult civilians in their territory? PROCESS INTO MUNCH!