Silinruls

Silinruls were a dominant lifeform in the southern part of the Milky Way (or at least the area of it accessable via the hyperlane) around ten millenia before Christ. They have conquered vast number of other species, establishing the Silinrul Empire that seemed to be impervious to all outside threats. It's armies and fleets were endless, it's resources almost infinite, and unlike the Mankind it possessed an FTL communication system.
Its end came from an inside. The combination of ever growing hedonism that slowly devolved their entire civilization. Their weakening grip over the servant species was reinforced with the powerful biological superweapon known today as Shoggoth Plague. When they lost control over it, they were wiped out from the face of the Galaxy - together with 98% of sapient species inhabiting their territory.

Basic Information

Anatomy

Their biological form is unknown, because they purposefully wiped all traces of it after their cybernetic ascension. Their post-ascension form was purely mechanical, in a way similar to Mankind's more cybernetic Dolls, though much more advanced and reaching archeotech level. This suggests that either the silinruls were humanoid to begin with, or decided during the ascension that such form fits them all (which pretty much tells us nothing).

Civilization and Culture

Average Technological Level

Silinrul achieved great degree of technological prowess, especially during their apex period. Transcendence was a great achievement, and so was the creation of Shoggoth Plague, despite how nonhumanitarian both were, but after the apex period their Empire entered period of slow decline during which most of said technologies were lost.
Silinrul warships are believed to use a modified strain of the Plague with heavy nanotechnological component that allowed them a great degree of self-repair without the need of crew to actually do something. Their computers were generally a few generations better than their current state-of-the-art cousins. After Transcendence the silinruls themselves could partially merge with the computer systems of their ships, causing a brief explosion of neural connection technology.
All of said achievements were already lost during last century of Silinrul history, due to their steady decline.

History

Silinruls rose to prominence around BC 12000, and soon conquered entirety of what is now known as Human Space. During their apex period (between BC 6500 and BC 6000) they western border was actually within the current yatevian space, while the eastern border incorporated current territories of insectoids and berserks. It is unknown why they seem to have never entered Solar System.
They enslaved or exterminated many species in an extremely cruel ways, all the way into selective breeding and gene/robomodifications to ensure docility and obedience. While their slaves were sufficiently pacified - with implants and genemodifications changing them into living zombies - the Silinruls lived in permanent fear of the Protectorates. The parts of space dedicated to hosting an umodified population of slaves species with breeding purposes. The modifications done to slaves was often bad for fertility, forcing the Empire to rely on marked breeding zones to supply it with enough slaves.
Their biggest trump card against the vast population of Protectorates that outnumbered the Higher Castes at least 10 to 1 was the Shoggoth Plague, a geneengineered bioplague that could devour biomass of entire planets almost overnight. Every planet that dared to rebeled was scoured of all live without mercy, with the rebels unable to do anything to stop it due to the Silinruls having absolute monopol on starships. And near-absolute monopol on heavier weaponry.
Around BC 6000 they achieved the imperfect form of Transcendence, by moving the minds of their entire species into a robotic bodies, and purging all remaining information on their pre-Transcendence forms. The machine silinruls remained around until the complete collapse of the Silinrul Empire around BC 5100, due to loss of control over Shoggoth Plague, Ancients expansion and a massive slave rebellion leading to the even known as Darkening.
It is believed that no silinruls survived the Darkening, firmly establishing them as an extinct species.

EXTINCT