Slarecians are an evil race of humanoids found in the Scarred Lands campaign setting for D&D 3 and 3.5 editions. I've long loved the mystery of these beings and how all that has been left behind by them either causes fear, kills without mercy, or sows discord. As I was starting work on my sci-fi setting, I knew that I wanted the first colonists on Mars to find that they weren't the first to set foot on the red planet. I wanted an alien race full of dark foreboding and mystery. The Slarecians are a perfect fit, and below is how I'm fitting them into my setting.
In 2036, the first Martian colony was established in a joint US and EU venture. A couple months after the establishment of Burrough's Promise, a surveying team found a strange metal arched doorway at the base of Olympus Mons. The metal was unlike anything that had ever been seen before. It was nearly impervious to harm and had an oily sheen not unlike what is seen when oil and water are combined. Completely by accident, the door was opened by one of the survey team. They retreated from the site and called in an archaeological team. Mission Control also had a security force go with the team to explore the opening. What was found was evidence that humans weren't the first species to visit and colonize Mars.
The site was dark and contained featureless walls of the same strange metal. Hallways created a maze like effect, but all seemed to eventually end in a central domed room. The central room was adorned with stone plinths and altars with strange, and as several members of the exploratory team recounted, insidious-looking runes carved into them. In the center of the chamber was a stone archway adorned with more of the wicked-looking runes. One of the researches touched a rune on one of the altars, which turned out to be control panel of some kind. The room was bathed in light from an unknown source; it seemed to come from everywhere. The runes on the central arch lit up as well. What happened next was not fully understood until after the Fraal made contact with humankind due to a total failure of the vid-comm systems each member of the team was equipped with. The bits that came through were disturbing to say the least. A number of creatures poured through the archway, which appeared to be a portal of some kind. Other than clips of screams, gunfire, and brief glances of the hunched, gaunt, humanoid monsters, nothing else was seen. Within seconds the life signs of all members flatlined. The security detail stationed at the opening to the site was ordered to cause an avalanche to block the opening since tests on the metal concluded that nothing less than a nuclear explosion would have much chance of affecting the metal.
The Olympus Mons "tomb" shocked the US and EU leaders; all of whom covered up all news of the discovery and the events that followed. It didn't take conspiracy theorists long to notice that a permanent UN military base was set up near Olympus Mons, however. Some of them even got close to what the UN was protecting/hiding. This disaster also colored perceptions of aliens by the US and EU, which is why relations with the Fraal were very cold and cautious to begin with.
It was the Fraal that provided the Earth governments with information that filled in the gaps on just what that site is. The Fraal had encountered other Slarecian ruins on other worlds in their history. What little they knew was that they were a seemingly long-dead Precursor species. All that remained of their culture was the ruins made out of what the Fraal called "Slarecian Steel". They culture, from what little they could discern was devoted to "esoteric sciences" like dimensional travel, torturous surgeries, horrific genetic testing, and psionic potential. Near as the Fraal had been able to piece together at the time was that they traveled the cosmos by way of the stone arches like the one found at the Olympus Mons site. The Fraal strongly cautioned the Earth governments to leave these sites alone. While the Fraal were also keenly interested in Slarecian research into psionics and would also like to know more about the history and culture of the species, every site, while different save for the existence of at least one stone archway and being made out of Slarecian Steel, is deadly beyond all comprehension. To this day, the Olympus Mons site has never been excavated, the defenses have only been strengthened over time.
Slarecian ruins have been found on other worlds the galaxy over, all of whom are instantly declared off-limits by any government ran by leaders with even an ounce of sanity. However, there are those that hear tales of untold riches in technology, science, or simply precious metals and jewels that tempt fate and try to explore a Slarecian site. Some of the horrors encountered are: Slarecian Ghouls, these twisted, savage beings are living creatures. Some scientists theorized that this is what became of the Slarecians, while others contest that they are likely the result of genetic testing or were specifically bred for defense. Either way, these creatures are the most commonly encountered threats in Slarecian ruins. Slarecian Gargoyles are automatons of psionically animated stone. Slarecian ruins are often adorned with statues of these winged creatures, as well as of Slarecians themselves, but only the winged statues are animated. They attack with intense savagery, but little cunning, unlike Slarecian Ghouls. One of the most insidious dangers is the Language Virus. This is a psionically spread virus that breaks down the ability of those affected to understand what anyone else is saying. So far, no cure has been found for the disease, and thanks to the ability for the virus to spread via psionic energy any who contract the disease, especially psions whose mastery of psionics only increases the transmission range, are either killed or simply left behind.
Many Slarecian ruins are destroyed by orbital barrages when discovered, so those that wish to try and profit from such sites typically have to work fast. There are credits to be made plundering Slarecian ruins even in spite of the dangers. Slarecian items are powerful in the hands of a psion and go for high prices on the Black Market (the sale of such items is banned by most stellar nations). Some psions have found chambers that when the correct rune is activated, allows them to expand their psionic potential with relative ease. Even Concord scientists would love to study the Portal Arches found at every Slarecian ruin site. The possibility of being able to unlock dimensional travel is hard to resist.
It is the possibility of dimensional travel that leads many to believe that the Slarecians are still out there somewhere, but only the insane would ever want to meet one.