Fifth Era of Humanity
The Fifth Era is marked as the era of human enslavement to the alien Mastermind AI. Almost every human in existance was directly linked to the Mastermind, who manipulated both the human race and specific human individuals on the subconscious level. Although it was capable of taking full manual control of any human's mind, it resorted to such actions only in case of emergency.
The Hive prospered. Everyone worshipped the Mastermind. A huge, modern fortress was constructed on Napoleon, that allegedly housed the Mastermind, who was thought to be a human creation to bring peace to the human race. Only a few chosen humans, the descendants of the original teams that discovered the Mastermind (or the same people themselves, now hundreds of years old) knew the truth - that the Mastermind is alien, extremely ancient in origin, and located in some barely explored system on the fringes of Hive space. The Fortress on Napoleon was of course nothing more than a ruse.
During this era, the human race greatly expanded across the stars. Eventually, the borders of the Hive reached 3/4 of the way to the Galactic Core, and extended thousands more light years towards the rim. The technological development soared through the roof.
The Hive was close to being a utopia, although it wasn't. A very small portion of humans was not implanted with link-chips. And they made it very clear that they will not rest until the human race is liberated from the evil slavery of the mind that was forced onto it. The Free Worlds, as they came to be called, were scattered and well hidden on the fringes of Hive space, or slightly beyond. As soon as the Hive stumbled upon a Free World, an invasion team of Gondorians, who became the ultra-elite shock troops for the Hive, was sent to conquer the planet and implant the population with link-chips. Should the invasion force be defeated (something that ever happened maybe a couple of times) the planet would be bombarded from space to oblivion.
Battle of Brakkar
A famous example of these was the Free World of Brakkar. A cold, freezing world with winds so powerful that they could crush skyscrapers like they were matches. The entire population lived in "upside down" skyscrapers - underground. The primary means of transportation were vehicles that could bury through the ground, there were even military versions of heavily armed mole-crafts that could reach all the way to the bottom of the planetary crust. When the Hive reached them, they fought, but soon their small defence fleet crumbled, the satellites of their orbital defense network were efortlessly blown away, and their ground-to-space missiles completely failed from stopping the wave of landships that screamed all the way through the atmosphere, landing Gondorian forces that quickly started working to destroy the desparate defenders. However, they failed. As a result, Brakkar was bombarded. Because the entire population was located deep underground, and many bases were multiple kilometers below the surface, the planet's crust was completely cracked open by the Hive forces, in order to absolutely ensure that the rebels will not survive. To this day, the twisted planetoid that once housed a population of millions serve as a reminder to the ruthlessness of the Mastermind and its human Hive.
The Ongoing Struggle
However, the Free Worlds did not succumb to despair. These centers of free population were very elusive - Brakkar itself was almost 40,000 light-years away from the center of Hive civilization, Napoleon. The remaining Free Worlds, space stations, space ships and others constantly harassed the Hive by raiding their supply lines and civilian shipping. Piracy was the main tool of the Free Worlds against the vastly superior Hive forces. While doing this, they did not forget their first and foremost goal. They bid their time in search of an oportunity to gain an advantage. Such an opportunity arrived at the year 5322.
After failing to intercept a convoy carrying military technology, a small rebel vessel was running for its life from a pursuing Hive vessel. The captain ordered a blind jump to try and evade the approaching enemy. When the ship disengaged its Maggot-drive, it found itself crashing onto an unfamiliar, Earth-like planet. A minute later, the pursuing Hive vessel that emerged back from the Hyperworld and into our universe has found itself in an identical position. Both ships managed to somewhat control their crash, but still, on the less advanced and safe rebel ship, several crewmembers have died in the crash.
The ships landed several hundreds of kilometers away from each other, so the primary worry for the rebels was to establish their location and surroundings. Soon they discovered that this planet was home to a primitive, but yet, sentient, alien civilization. The crew was not surprised. Dozens, if not more, sentient aliens have already been discovered by their time. However, after exploring some more, they have stumbled upon a new alien presence. For the first time, a human had the opportunity to look upon an alien race approximately as advanced as itself.
War of Eradication
Soon, Hive forces arrived on this place in search for their lost companion. They quickly found out about the new alien race. The Mastermind percieved them as a threat, and a war started. The insectoid Knoth were helpless against the vastly superior technology and resources of the Mastermind-directed Hive. They fought bravely, but star after star, sector after sector, they were losing space and their borders came to be dangerously close to the homeworld.
The Rebels Strike
The rebels, taking advantage of the Hive's distraction, chose this moment to attack. Up to this day, historians don't know what exactly happened at this point. However, two facts are obvious. One: the rebels have suceeded at their great goal, and the human race was free. Two: If you go to Pandora's coordinates today, you will find a star, and in its orbit there's absolutely nothing. Not a trace of a planet, or an alien construct, or anything of the sort. Some people think that the coordinates obtained from rediscovered records are flawed, others think the entire Hive is nothing more than a myth. Whatever is the answer, it is clear that the human race of that era was very powerful, and united.
With the Hive gone, the humans had no reason to finish off the Knoth, and a peace treaty was signed. The Free Worlds started celebrating, but their celebrations were way premature. A mere week later, the intelligence agency of the newly established government, that was so fresh that its name wasn't yet decided, recieved indications that the Knoth are being subverted into a Hive of their own, just like the humans were. It was unclear how exactly it was carried out, or who was behind this (another alien AI?), but the undisputed evidence were there.
And then, before the humans had the chance to decide on their course of action, the Onslaught Fleet arrived. Alien vessels suddenly appeared in human space, and began wrecking havoc in their path. Nobody knew who they were or what they wanted, but their responses in combat hinted that they were also, yet another Hive. They were powerful beyond anything the humans had, even with all the accumulated new technologies over hundreds of years under Mastermind rule. The performance of the alien ships spat in the face of physics, their weapons were unstopable, their armor - impenetrable. The humans won the Onslaught War, but only because the Onslaught fleet was very small. The fact that two thousand human planets were now two thousand asteroid belts, however, reminded the extremely weakened and crippled humans that their victory was nothing less than a miracle.
The human fleets, now barely deserving the name flotillas, were scrapped to provide housing and power sources to the now starving human populations. Billions of humans died from hunger as the infrastructure collapsed. The newly established General Galactic Alliance tried to correct the situation. But since it couldn't do much more than to sign papers and make declarations, the situation did not improve, and people generally ignored its existance. Soon, pirate groups have formed to ravage some planets, and selling the stolen food to the highest bidder among the starving, powerless planets.
The Dark Age of humanity has begun.