Extinct systems

Since the Amnesiac Virus, not all systems discovered and colonised by humanity survived. In the years after the unleashing of the virus, some worlds succumbed to one of the many perils that can face a civilisation. Most reasons for this are mundane - the ever present threats of war, famine and collisions with asteroids and so on, especially risky for those colonies that lacked a large biosphere to fall back on, and were perhaps limited to space stations, asteroid bases, or airless moons. Some extinctions were less mundane, some are considered so mysterious that the system in question is effectively under quarantine, lest whatever caused its population to disappear could spread. This is a record of the most notable.
Slueders Sector 4711 Remote surveys of the primary world in this system indicate that it was one of the more successful colonies in human known space, even after the Amnesiac Virus, with a large population and highly advanced technologies. At some point after loss of contact with other human systems, the entire population of both the primary world, and all nearby colonies died out due to an as-yet unknown biological factor. This agent was so virulent, it appears that it has not only killed off all advanced lifeforms, but also all life, right down to the microscopic and viral. The world sits eerily still, with all life on it now quiet and undecayed, in some kind of mockery of a living ecosystem. The Federation of Xah has placed a wide cordon around the world, forbidding any approach, in case whatever killed it off is capable of further transmission.
Zoen Sector 3409 Another once bountiful colony, the human population is now unrecognisable from the people they once were. Some form of mutagenic agent affected the main planet, and the remaining inhabitants are feral and exceedingly dangerous, with heightened speed, agility and strength and seeming biological immortality. Thankfully their intelligence has suffered as well, restricting them to their gravity well. The mutants are hostile to anything but themselves, leading the Northup Alliance to quarantine the world after several missions sent down to the planet all failed.
Geyot Sector 2555 This was a binary system, with one main star with a rapidly growing cosmopolitan population, and the minor star housing one of the most advanced research stations in known space, employing some of humanities best and brightest. When the Amnesiac Virus hit, records indicated that the larger colony suffered tremendously, but a quick-thinking government managed to retain control and was on course to prevent the loss of the colony. At an unknown time after that, the gas giant around which the scientific station was orbiting exhibited a sequence change, turning first into a yellow dwarf star, and then collapsing into a pulsar. The pulsar emitted an x-ray beam of significant strength that bathed the nearby colony with vast amounts of radiation for several years before abruptly vanishing, taking the scientific station with it. By this point, the entire population of the colony, along with most of the multi-cellular life on all of its worlds was dead or suffering from prolonged ailments due to the intense radiation. The remnants of the population that escaped the fatal radiation dose died soon after from other diseases and starvation. The system has since been partially re-colonised by the Northup Alliance as a military base, but instances of cancers are higher than average.
Istrol Sector 2347 One of the most distant stars from Sol, this system was the only known location of a corporate weapon testing facility designed to look into weaponising Krasnikov-Hiraski Event technology. The facility was built as a moveable ring station so that it could be moved close to any Keyhole of interest. At least six Pioneer Vessels had been dispatched from the system, testing if the relativistic nature of the journey could be used to launch kill vehicles through the wormhole, bypassing the otherwise impractical fuel requirements. Like a lot of weapon testing facilities, critical systems were managed by an AI with a suspicious and paranoid nature towards those it did not deem valid occupants. The AI survived the Amnesiac virus, unlike almost all of its like, but was damaged, and came to view the humans under its charge as hostile and wiped them out. Thankfully, operators at the facility managed to damage the AI's capability to move the station, however it still retains control of its highly effective defences, and some measure of ability to aim the Keyholes. As such, the system is unapproachable, as the AI targets any incoming ships with relativistic kill vehicles of substantial power. Shots have also been fired at nearby inhabited systems, stopped before they caused any damage, but the fear is that the AI is on a mission to kill all humans, and only its inability to fire 'dark' shots are keeping it from fulfilling its self-appointed task.
Gahloph Sector 371 Whilst not the first system to be discovered with alien lifeforms on it, or even the first with complex life, the moon orbiting a gas giant in the system gained attention due to the extreme nature of the ecosystem. The gas giant was well within the 'Goldilocks Zone' of the nearby star, and the moon's surface had abundant water, but its orbital period and distance from the gas giant led to 'bands' of either extreme cold or heat, with a temperate zone that slowly crept across the planet. Life in these zones flourished, with life there either constantly on the move, or hibernating during the extremes. Research stations sprung up over the world, and it became a popular place for exobiology research and even gained notoriety as a tourist resort, with large mobile hotels creeping along in a zone of a specific climate. When the Amnesiac virus hit, most of the population starved or died as the mobile buildings failed and were consumed by the oncoming winter or summer. If this was just it, then it would be chalked up to yet another unfortunate victim, but it was the fate of those who survived that have led to its inclusion on the list. When the Federation of Xah arrived in the system, there was evidence that whilst some of the population had survived the initial disaster, something unknown had destroyed the remaining population some hundred years later in a particularly destructive manner. Exactly what is unknown, but the prevailing hypothesis is that the moon experienced an abnormally hot summer that managed to melt the usually consistently frozen ice caps, with previously unknown organisms being released from their hibernation. The scale of destruction of the settlements implies large creatures, and not something microbial, but no evidence has been found of any creatures large enough to cause the level of damage seen. The Xahians have placed a mortarium on re-settling the world until the mystery can be solved.