The Sedulthra are a hostile alien race that were introduced in the future Y3K Universe.
Sedulthra are a silicon-based life form that evolved on a high gravity, high temperature, and high radiation world. The resulting harsh environment led to the evolution of a creature that is incredibly durable. The average Sedulthran's body has the tensile strength of a diamond. Some believe they are living nuclear furnaces, similar to the Bio-fusion Bug. They feed their growth process through massive internal energy generation that is used to form the crystalline patterns that comprise the Sedulthran body. Sedulthrans can also focus this energy and project it as raw microwave energy.
Due to the massive energy needed for a Sedulthran to live, their bodies generate such high temperatures that they cannot exist on most worlds due to the difference in temperature between their environment and inside their own bodies. In other words, they can only live on planets far, far too hot for humans to live on.
Contact with Humanity
In 2256, an Earth Fleet deep space exploration vessel, trying to determine just how far the Bugs had spread, was deep into the Arachnid Quarantine Zone beyond the galactic southwestern border of mapped space when they intercepted a coded alien communication. Encryption experts were unable to decipher the code, but were sure the communication wasn't from the Bugs, but a previously unknown alien species. Eager for an ally against the Bugs but cautious and suspicious after the Vin Shriak Holy War that had only recently ended, the Earth Federation launched several small stealth incursions into uncharted space beyond the Quarantine Zone to gather intelligence. On several instances brief radio contact was made, although Earth Fleet was never able to decipher the alien language. After a year of failed attempts to locate the aliens or even achieve direct communication, the Federation abandoned the incursions but warned all Fleet captains to be aware of potential hostilities from an unidentified alien race.
Seven years later in 2263, a rogue Bug outpost was identified in deep space when a passing probe detected strange communication signals emanating from the planet. They didn't correspond to Bug signals, but a second probe did confirm that there was an Arachnid colony on the surface. Earth Fleet sent a long-range scout ship to gather more information. When it arrived, however, the crew detected three large unidentified vessels orbiting the planet. Scans indicated there was a major battle being fought on the surface below. Again, the aliens made only brief radio contact in an unintelligible language. Not wanting to risk starting another conflict while the Federation was still licking its wounds from a over decade of warfare, the scout ship withdrew.
No further contact was made, but Chairman Clarke wasn't too worried as the entire vast Arachnid Quarantine Zone lay between humanity and whatever mysterious aliens were out there. The next year, however, brought the chaos of the Caal Invasion and the collapse of the Federation, the Ascension War and the birth of the Holy Terran Empire, and the vast reduction in travel and trade due to the dismantling of the hyperspace jumpgate network. With humanity now more concerned with rebuilding civilization than spreading it, deep space exploration under the Holy Terran Empire effectively ceased.
Humanity did, however, gradually push into Arachnid space over the next few centuries, slowly reclaiming worlds and colonizing them. Meanwhile, the Sedulthra were doing the same thing, but from a different direction. Six hundred years later, in 2814, the two fleets bumped into each other, their language translated and communication achieved, and official first contact was made with the Sedulthra.
It did not go well. An aggressive and militaristic species, the Sedulthra considered the Imperial Navy to be trespassing in Sedulthra territory. Although a conflict was narrowly avoided when the outnumbered Sudulthra ships withdrew, they apparently nursed a grudge against that humiliation as a tense cold war quickly arose along the border of the two species. Consequently, humanity still knows relatively little about the Sedulthra or their technology.
Several small colonies on the edge of human space were destroyed over the next few years, but as there were no survivors the Empire has had a hard time proving the Sedulthra were responsible for the raids.
Behind the Scenes
The Sedulthra were ceated by Nathan Bax for his Y3K game, almost certainly inspired by the Tholians from Star Trek. They were mostly just background window-dressing for the Y3K universe, mainly there to provide humanity and the Empire with an external threat. These aliens have never actually appeared in the tabletop games or story cannon, and very little has ever been established about them. They're a martial race and highly aggressive, both militarily and intellectually, but other than that their society and culture is wide open to future player/writers to develop.
Almost all details in this entry come from Bax's TI archive website, although retconed to be more compatible with recent developments in the TI Universe.