The Dark Millenium is the commonest name given to that era in Arth's history when humanity was almost wiped from the face of the planet. Scraps of legends and ferasome rumours are all that have survived from this period, but sages wise in ancient lore do agree on several points. During the Dark Millenium, the world was invaded by powerful beings from another plane - or perhaps several other planes. It is not known if these creatures created the portal themselves or took advantage of one opened by a careless mage. In any event, the result was the same - an eruption of monsters into the world. Foremost among these were the capricious creatures known as Fays or the Liasssifar and Dverige, but there were also demon lords, vampyres and other creatures unnamed. Warminsor the sage wrote that an ancient Liassafar Lord told him their race had come to Arth through the portals to escape many of the monsters that followed and that his race's fall was more due to their ancient foes, than humanity's efforts.
Unable to tolerate the light of the sun, these creatures, led by potent sorcerors called the Dark Lords, created mighty magics which blocked the sunlight, casting even full noon into gloom. Under this pall, they crushed the human kingdoms so thoroughly, that even their names are lost. There were only a few exceptions. The ancient and mighty empire of Thanataya survived, although many of its cities were destroyed, and others only existed as networks of tunnels beneath the ruins. Although most of the star castles fell, others escaped the taint. And the cities of Lune and the Redworld were largely unharmed, although they survived only by cutting their ties with Mother Arth.
However, after the first onslaught, humanity gradually began to recover its lost world. The invaders - initially united - began to quarrel, and fight among themselves, in some cases even employing human warriors, albeit horribly warped by magic. The invaders also brought with them creatures of fable from their own world, whose descendants now haunt Arth's nights, while bereft of sunlight, many of Arth's own creatures and plants perished. Only the strong and the savage endured outside the domain of the few free strongholds.
The desolation of those times can be read from a fragment of verse in the ancient declamative mode, whose author is unknown, about the fall of the city of Pahizun. Today, no-one can even name the kingdom where Pahizun stood.
Ah, Pahizun! Your white walls stained with smoke!
Ah, Pahizun! Your gold-roofed halls desolate to the sky!
Ah, Pahizun! Foul darkness couples where ladies once gave benison!
All evil gods rejoice! Pahizun is fallen!
Ah Pahizun! The crystal waters sewer'd with bones!
Ah Pahizun! Ancient and youth but meat for rats!
Ah Pahizun! The sword of Ilonquai broken!
All good is fled! Too great this loss for weeping!
Eventually, of course, the darkness was beaten back. But traces remain. The Land of Shadow, to the south of the Deserts of Lamentation still lingers under sorcerous darkness, and foul monsters still slink from its fringes. Many a ghastly creature still lurks in desolate wastes, or creeps by night, whose ancestors were spawned in the Great Darkness. There remain too, potent and evil sorceries in the lost places of the world, that persist undying. And the Liassafar and Dverige still hold their hidden halls beneath the hills.
The Dark Millenium has left its trace on humankind too. It is said that the race of Ogres were bred for soldiery by the Dark Lords - although others say they were bred in that same era by the sorcery of the Princes of Lune. So few records survive now, that we will likely never know the truth.