"They were called the Mrevani. Ruthless, vicious, bent on nothing but conquest, and worse..."
The Astro-Naut witnessed them destroying a whole star system. Four worlds, two moons. He engaged them in battle. It is unclear if he was successful, or if the devastation was so vast, that even in victory, it seemed like a great loss.
He encountered them near Procyon, a star system, about 11.5 light years from earth. From what he learned of their history, they had barely managed to civilize themselves when they were visited by interstellar explorers. They murdered them and took their technology, using it to take to the stars themselves, making war on everyone they encountered.
Eventually they found their way to Romeyn Falls and Earth. On a late Spring night in 1946 numerous Mrevani vessels blackened the skies. The world was still recovering from the aftermath of World War II. Despite that battle weariness, it wouldn't have made any difference. Earth wasn't ready for alien invasions in that time frame. They didn't have the technology, or so everyone thought until, like the cavalry in a Western Epic, the Astro-Naut returned from his self-imposed seclusion. Accompanied by a fleet of futuristic drones that he had spent years producing, he once again engaged the evil foes. He made it very clear to the warring Mrevani, "Not this planet. Not this city."
The outcome was a complete eradication of the entire Mrevani invasion force. Their fleet, even the mother ship destroyed. Nothing survived the great explosion in the sky. Nothing for government agents to scavenge or try to replicate for their possibly devious goals.
Their actual place of origin is unknown. Their total population not revealed.
Are there still Mrevani out in space somewhere, plundering other alien civilizations? If so, how many more of them... waiting, wondering what happened to their warrior brethren who went missing on what was thought to be a minor mission to Earth. Perhaps they're preparing for a return visit in our not too distant future.