No moment in the history of the Terran Expansion is as shameful as humanity's first encounter with an alien civilization, the Maltruskan Union. The Maltruskans were a gracile hominid race that had settled many worlds near Sol. Their society was mercantile and clan-based. Their technology was a stable Imperial Standard T+2 on the Diasporic Scale of Galactic Development, and they made good use of the ancient network of slipknots, as well as of hyperspace travel.
The Mastruskans' Fall after their first encounter with humanity was precipitous. The problem was both biochemical and psychic. Biochemical: the Maltruskan's were vulnerable to a form of pheremonic attraction and servitude to humans. Psychic: the Maltruskans were moderately empathic, and overly responsive to humans' appetitive nature and the instinctive psychic pressure that humans exerted in proximity to Maltruskans.
Of course, the problem was also imperial: the humans of the Terran Expansion era simply didn't care very much about the welfare of even very similar species. They viewed the Maltruskans as one more resource to exploit to Terran advantage.
The result was the wholesale disintegration of Maltruskan society: their mass enslavement on the humans' plantations and in their extractive industries, and the destruction of the Maltruskan's household, clan, and inter-clan systems. Within a few years of their initial contact with humans, Maltruskan society and technology was in ruins, and only a few enclaves of an odd, subterranean troglodytic Maltruskan sub-species remained extant.
Things did not fare well for the Maltruskans' new masters, either. Once the Maltruskans' network of settled worlds had been overrun and incorporated into the Terran Expansion zone, human expansion stopped, and human-settled worlds became ever more culturally involuted, decadent, and ineffectual. Some Imperial scholars have suggested that this was a hidden vulnerability inherent in the empathic bond established between human masters and their Maltruskan slaves. Whatever the case may be, the end of the Terran Expansion Era came swiftly in the form of an invasion of Comet Barbarians from the periphery of the Terran Expansion zone.
One of the Comet Barbarians' chieftains became the first Imperial Sovereign. Glorious First conquered all the worlds of the Terran Expansion zone, and she began the forcible Separation of the human and Maltruskan populations. The second and third Imperial Sovereigns completed the dismantling of the traffic in Maltruskan flesh - which extended into the barbarian zones well beyond the formal Terran Expansion zone. These two Sovereigns abolished slavery of all species and kinds within the Empire forever.
The fourth Imperial Sovereign, also known as the Banisher Pontiff, carried out a decree of Excommunication on several Maltruskan worlds that had been purged of human populations. In this way, it was hoped that the Maltruskans might rebuild their culture and civilization, even if limited to a few isolated systems.
Notwithstanding that, there continue to be Maltruskans out and about amidst the Empire. A brave few make their way among humans by using hermetically and psychically shielded encounter suits. Still others have established small clans on remote or barbaric worlds. It is said that a few Maltruskan tribes even wander the deserts of the Imperial Throne World of Altair III. Still others - perhaps the troglodytic psychics - are said to have taken refuge within the lower levels of the Imperial Palace itself.
More than a few humans, meanwhile, dread the day when the humans of the Empire have that inevitable disastrous first encounter with a species for which they are not biochemically or psychically prepared. If this disaster befell the starfaring Maltruskans, why couldn't a similar one befall humans themselves? Modern Imperial Quarantine Procedures are based on the assumption that one day this will happen.