The Darktide is the name that the Imperial authorities of the Moebian Domain apply to the shadowy xenos and Chaos forces on the domain's borders, the so-called "Fringe," that threaten its worlds.
History[]
The Moebian Domain is highly loyal to the Emperor and the domain itself is considered a significant tract of Imperial space. The Moebian Domain is a fiefdom comprised of several important worlds, loyal to Terra and ruled over by Lord Constant Margrave of the domain's capital Hive World of Atoma Prime, who serves essentially as its sector lord with the title of "Lord Moebian."
The domain can be considered a province or a self-governing state of the Imperium. It pays its tithes and taxes to Terra, it obeys Imperial law, and its worlds manufacture vast quantities of strategically important goods for the Imperium. It is thus highly important to the Imperium and must be defended from assault by the Emperor's foes, whether they be within or without.
Unfortunately, the Moebius Domian borders several disputed territories of Wilderness Space called the "Fringe" that contain multiple xenos and Chaos threats. The Adeptus Administratum ensures that for the average citizens of the domain the nature of that threat remains a mystery. It is just called the "Darktide," a shadowy threat that must be kept at bay if the people of the domain are to remain in the God-Emperor's light.
It is to protect the domain from the Darktide that the Moebian Regiments of the Astra Militarum are raised, mostly from the teeming populace of Atoma Prime but also from several of the other worlds of the region. The Moebian Regiments have for centuries been fighting an on-going conflict known as the Fringe War against the Darktide. These regiments were also ignorant of the Darktide's true nature, until they began long tours in the Fringe War and finally faced the Moebian Domain's nightmarish foes.
Long tours result in trauma, both physical and psychological. Some "lasmen," as the Moebian Regiments call their guardsmen, are broken by the experience. Some are made bitter and resentful that the gruelling combat they experience is generally unrecognised back home, because the Administratum tends to keep all details of the ongoing conflict quiet so as not to generate public alarm. There might be a few news broadcasts about "great victories," but the rest of the turmoil is screened by propaganda.
Some troopers, of course, serve brilliantly and bravely, and remain loyal despite everything they experience. But others fall prey to the insidious touch of the Darktide. They encounter the malevolent effect of the Warp, and it changes them. They become -- literally, in some cases -- the very thing they are fighting against.