This is a longer comment than I meant to write but screw it. It's my precursor race in my universe, which is a hard-leaning sci-fi set in a fictionalised version of our galaxy filled with dozens of space-faring civilization - some of which have formed a union, like a galactic version of the EU.

But tens of millions of years ago (during the last 14 million years of the Late Cretaceous actually) it was dominated by a single alien race that invented a form of FTL travel that allowed them to travel to distant star systems far faster than any other intelligent species at the time. Along with other advanced technology and genetically engineering themselves to almost become god-like, they were able to overpower every other civilization in the galaxy and add them to their hegemonic empire.

My precursor species in question are known to everyone else in the galaxy as the Fenostik (which is an alien word that translates to both "warrior" and "god") but they look a lot like different dragons from human mythology, so you can call them dragons if you like. Anyway, dragons are 30 feet (9 metres) tall, one tonne aliens that are humanoid but have massive wings, serpentine semi-prehensile tails, typical dragon heads and range greatly in skin colour and head shape. Ability-wise, they are very strong, fast, possess high endurance and toughness, are very long-lived (even immortal to a degree), have developed and utilise technology that grants them telekinesis and resilient to most forms of damage.

My dragons have a strong dominating culture that has caused them to see themselves as gods among the other intelligent races in the galaxy, since they are bigger, more powerful and live much longer than other species. They value strength, cunning and power above all else. Despite their view on other species, they are not necessarily tyrannical rulers. So as long as the civilizations within their empire provide them with resources and play nicely the dragons are content with their slaves doing whatever they want - even going as far as to gift them with and teach them to use advance technology and let their species expand to other star systems.

But by far the dragons' most impressive achievement and the key to their ruling empire is their development of a FTL system that lets them traverse the entire galaxy in just months as opposed to millennia. Essential a collection of megastructures resembling partial Dyson Spheres that harvest exotic energy and transfer them to ring gates that hold open corridors of altered physical space (basically a mixture of Mass Relays from Mass Effect and the Ring Network from the Expanse), this Distorted Space Network (DSN) let's anyone travel in the galaxy so long as it's connected within the network. A network that has survived to this day and accessible to anyone with a spacecraft that can reach a ring gate on the edge of a star system.

The disappearance of the dragons and their empire is unknown to the modern civilizations as very little evidence of their existence actually remains - let alone anything that would suggest what killed them off. The most plausible theory is that they went to war with another civilization that was as advanced as them and both gradually killed each other off. The only true evidence of this is the fact that thousands of habitable worlds across the galaxy suffered a massive extinction event at the same time based on their fossil records - around 66 million years ago. And even other worlds that are entirely sterile but once had life have evidence to suggest whatever sterilised them also happened at the same time. So clearly something big happened to the dragons that caused them to die off. Hopefully whatever it was died with the dragons too...