Lovecraft's Elder Things

The creatures called "Elder Things" are ancient beyond words. They may have been the first species to evolve intelligence in the Milky Way galaxy, and as such they trace back a history easily six billion years. They normally wander the interstellar gulfs in ships of such advancement and sophistication they are night indestructible and they trade knowledge for their own strategical puposes. They haven't used FTL propulsion the last few hundred million years, apparently "cautious with lies between" and thus take their time traveling at a few % of light speed.

In the 25th century an Elder Thing vessel passing the solar system was strangely and mysteriously rerouted and sold a batch of FTL generators to humanity on credit. At that time humanity was enmeshed in its first interstellar conflict and it was losing. With that technology humanity won from its adversaries and was able to establish a many thousand year lasting peace.

In some arcane past literally hundreds of millions of years ago, Elder Things did in fact live on Earth. From fossile evidence it's clear their night indestructible bodies have changed remarkably little in that time. They were a fair bit more expressive back then and they took part in several major aeon lasting wars, primarily against Deep Ones and Flying Polyps.