When mankind ventured out into space, we never suspected what was to come. Our first inhabited planet was less than thirty light-years away, far closer than we would have thought. The issue?
Everyone was dead.
The entire planet was like that, a perfectly stable world with no issues that we could determine, and yet it seems the suicide rates were the greatest cause of death among the people. We don't know why, but they apparently had been doing this for some time. Years of study later showed that the reptile-like race had taken nearly ten times as long as we did to reach the industrial age, and had not gotten very far beyond that.
Time went on, and we soon discovered that this was not an isolated occurrence. Species after species had killed each other and themselves off for one reason or another. Some had died off so early only a few stone monuments marked that they had ever been there, some had established empires spanning nearly a dozen systems. But always they were all dead, down to the last child.
And so we traveled the stars, colonizing the lost homeworlds we found, along with others. We studied the creatures on many worlds, although none bore intelligent life. In time we studied the technology of the more advanced dead races, gleaming a scientific secret or two we had missed. It was rather amusing to see the scientific community collectively slap their heads when they see the simple ideas and concepts they had missed.
Once, and just once, we found a sentient race still alive. On a planet with 3/4th the gravity of earth, a planet primarily dominated by jungles bore a race of insect-like scavengers. They were barely beyond the Stone Age, farming and agriculture still in its early days.
We considered making contact, but in the end decided against it. They were a very violent race, and many would kill one another for the smallest detail. Suicide also seemed to be something they would resort to with little or no good reason. In fact the entire race seemed insane. Long-term observation showed that after hatching the individual would slowly but surely go insane, to the point where none reached old age. In fact the only reason this race had not yet gone extinct is a combination of high birth rates, short life spans, and a child being able to defend itself hours after birth. In any other race this bizarre affliction can and did drive them to extinction.
For nearly seven hundred years, mankind grew and expanded. We did not find another living race during that time, or find out how we were immune. Many came up with theories, but none fit better than any others. Ethnic, religious, and cultural differences became less important when you were away from Earth, and in the end those who could not agree simply lived on different worlds. The UTA, United Terran Alliance, controlled over 87% of mankind's colonies from its seat of power on Earth. A few rogue factions cropped up, piracy and smuggling saw a rebirth in this new space age, and mankind went on as it always had.
Then a moment that would change our history came. The UTA Dreadnought "Supremacy" was in essence a city in space. Constructed with our most advanced technology, to the point where systems were updated during construction, and having a length of nearly seven miles, it was the mightiest ship we had ever created. The Supremacy was sent to investigate a new world our long-range sensors show had space-age technology upon it. It was farther out than we had ever traveled before, but not by much. It was assumed that this would be just another dead world, that we might find some usable raw materials pre-harvested in the form of abandoned constructions in orbit.
What we found was an outpost of a space-faring race, its people still alive and well. Dozens of ships were in orbit around a rocky empty world, along with a space dock to repair and refuel them. They were remarkably primitive compared to our own, and much smaller. The largest was perhaps the length of a football field, if that. From the scarring and damage, along with the derelict ship floating nearby of a different make, they had been fighting not too long ago.
Our first contact did not go so well, however. We would later piece together what happened, and it went something like this. As soon as what might have at the time been the largest spaceship in the galaxy appeared on top of them, the race known as the Kondar were sent to the edge of panic. The commander tried to keep a disaster from occurring and ordered his ships not to open fire. Communications on both sides failed, we ourselves had long ago stopped carrying any equipment or training our crew for first-encounters, and apparently the Kondar had not had any first contacts of their own in hundreds of years. It also didn't help that our subspace communications were just advanced enough that the garbled words the poor Kondar picked up on their outdated systems sounded horribly sinister and alien even by the wide standards of the galaxy as it now was.
One of the Kondar gunmen on the closest ship had a panic attack after hearing our garbled transmission. He fired upon the Supremacy, which in a placating gesture had lowered its shields. The shot was able to breach the hull at a single point, and cost the lives of three crewmen. The captain of Supremacy ordered the shields raised and a warning shot fired. Unlucky for the Kondar, the concept of warning shots was alien to them, and they did not stop to ask why the single shot had missed.
It was a short fight. The Supremacy blew away a single craft to secure its escape victor; a sad but necessary tactic. This only hurt our reputation further, with what happened later.
As we soon discovered, nearly 3% of the galaxy was part of what was known as the "Veil of Madness". Any race within this sector of space would slowly but surely go insane. Short jaunts were safe enough, but more than that and permanent damage to the mind would result. We had apparently been sitting in the galactic equivalent of the Bermuda Triangle, and had finally breached its edge. When the Kondar realized that both our entrance and escape vectors lead directly into the veil, a panic started. With surprising speed, news spread among their people, and among others, of what had happened.
The story grew worse with each retelling, especially once it left official military reports. Tales of the titanic black ship that came from beyond the veil, sent out signals in a horrid dark language, obliterated dozens of the Kondar's finest warships in seconds for no reason, and then vanished like a ghost soon spread everywhere. Humanity had become the bogeymen of the universe.
Raids from pirate groups further cemented our dark reputation, and in time we came to work with the role. Every attempt to convince people that we wanted fair negations was seen as a deception. Rather than fight a losing battle, we played to the role given to us. Soon we were seen as 'wicked but not unreasonable' and gained both fear and respect throughout the galaxy. Few humans appeared in view of aliens outside of deliberately frightening power armor, and human ambassadors used voice synthesizers to sound like that first garbled communication had.
Looking back, it's actually worked out in our favor. After years of contact, most alien races know almost nothing about humans other than exaggerated horror stories; they rarely bother us, and the ones that do never return home.
The only bit of info we were more than happy to share with them was the reason we can live in the veil. Turns out we were all a little crazy to start with. I think the fact that we're pulling the largest practical joke in the galaxy was already proof enough of that...