Mad Queen Incident refers to the series of events when the Librarians almost lost their control over the Great Library around 5000 years prior to the rise of Eremidia Kingdom. The events occured briefly–around 6 days (according to some Librarians' records about the history of Great Library) or 8 days (according to few survivors of the incident, including Nuloa)–but the impact upon the Library was immense; it crippled most of the Library's wings and corners and the casualties, either beings or goods, were high. Most records mentioned a massive troop of around 40000 personnels marched directly from the forbidden realm of exiles towards the Lux Primus, laid waste upon The Way and the Great Library, and then suddenly everything stopped.
Such fast and devastating strike from the then-unknown corner of The Rift becomes one of the Great Library's darkest moments, however Librarians, despite the invaders' treatment upon their kin, erected a monument and dedicated a corner in the Great Library as a “symbol of chaos led by sheer curiosity.”
Decades after the incident, Librarians managed to figure out the identity of the invaders and their leader. They were the lost kin whose ancestors were exiled from a realm in Kytheros cluster and sought refuge in a newly formed realm, leaving their civilization and technology along their way to said realm. They named their kin “Highvalean”–taken from the name of of their biggest city and highest civilization.
Highvaleans were avid and powerful artisans who can control the essence of power that creates the realms and realities: the firstmost and the most powerful kind of Ether, dubbed “Worldsmith's Tool” or simply “Tool.” Their last ruler, Agatha Lou a.k.a. the Mad Queen, was the craftiest and the most cunning of her kin who's capable to bend massive amount of Tool and eventually crafted a makeshift portal directly into The Rift–such act was the reason behind the influx of Highvalean from their realm despite The Way's connection between their realm and Lux Primus at that time was completely severed.
Since the first day of the invasion, the stability in The Rift became completely tipped, causing frequent ether storms and destructions of few Ways, rendering Riftwalkers unable to travel into certain realms and/or pathways. The Way itself changed drastically, making the landscape into “permanent night” and everything that used to be present along the pathways either destroyed or vanished.
Alongside the casualties made in the days of invasion, some new hostile creatures emerged in The Way, attacking the invaders and Riftwalkers alike. These creatures are still present to this day–in fact, no one knows how to remove them from existence. Riftwalkers called them “lurkers” due to their nature: stalking and lurking along The Way, waiting for unwary preys.
In the last day of the invasion, Agatha Lou was killed by one of her guardians and ended the invasion altogether. In the same day, Great Archivists arrived into the Main Hall and invoked one of incantations taken from Forbidden Chamber, directed to the entirety of Highvalean. The spell's effect makes almost entire Highvalean banned from entering the Library and removed them from history, effectively erasing their civilization and themselves from existence. The incantation was subsequently removed from a few Highvalean such as the spared ancient and the mirror image of himself.
In the aftermath of the invasion, Great Library suffered a lot of damage and casualties: hundreds of thousands of books and records were destroyed, thousands of Librarians were killed, and half of the Library's structure was severely damaged. The breach inside the Great Library–where the Forbidden Chamber lies–caused a shift of reality, creating an empty space inside the innermost Library room as well as “memetic hazard” that causing anyone who comes near the source of anomaly cannot think or remember what they have seen at their perimeter.
There is a corner dedicated to Mad Queen, where collections of curios are stored and sorted. Curators maintaining the corner and visitors who stumbled upon the corner shall pay attention that despite the immense knowledge the corner may possess, there lies a great danger that may be destructive for anyone and everyone around it.
Some rumors stated that Highvaleans weren't actually removed from existence, instead they were turned into something else; they become malevolent beings that eternally wandering around The Rift aimlessly. Since the first day of the invasion, one of these creatures emerged from nothingness as one of the Highvaleans fell in the battle. This theory has became a dispute among several Librarians, and the records that supported it are contradicting each other to the extent it becomes nearly irrelevant.
Despite rumors and disputes among Librarians, the erasure of Highvalean didn't erase their home realm since they weren't actually originated from there. Instead, a new Way formed out, connecting the realm and Lux Primus, making the realm becomes one of the Library's network. However, the realm's natives didn't figure such access was granted until a great quest against the inevitable happened.