The Great Enemy, the Infernal Legions, the Dark Ones, the Dalkhuri, or more simply "demons" or "monsters", the names given to these extra-dimensional invaders are many. Their origins are unknown and hotly debated. Some say they are the aftereffects of Creation, a natural balancing act by the universe towards the Divine Light. Some say they are the products of a second Creator, an “Anti-God” and the universe in all its dimensions are a combined creation of two diametrically opposed elements, seeking to determine which is stronger. Still others argue the darkness is a necessary component of the Divine Experiment, meant to ensure the creations of good are properly challenged and tested, forced to excel or vanish.
Whatever their actual origins, they are the greatest existential threat to mankind, likely even greater than man itself. They seek to enslave, consume, and destroy all life, and even inanimate matter is twisted and corrupted by their presence. The mere presence of the Great Enemy causes all physical matter around them to slowly warp, as if reality itself crumbles around them. While the lesser beasts and soldiers of the Great Enemy are merely mutated lifeforms (or even humans), the more powerful demons can wield powers that defy science and rationality. Conjured energy, impossibly fast reflexes, telepathic communication, opening smaller portals between universes, negation of physics itself, these are but a sampling of the abilities displayed by the greater demons.
And still even greater monsters have been fought, entities of planetary size with powers that would classify them as gods in older ages of history. Existence itself is but a simple material to be reshaped and altered to their whim. Although humanity has managed to slay even these titanic entities during the Judgment War, the tides of darkness are unceasing and unrelenting, threatening to drown our universe in everlasting night.
The Great Enemy hails from another plain of existence, though whether it is a separate universe from our own or simply another part of ours normally inaccessible, tucked away in alternate spatial dimensions from the ones we exist in, none are sure. But they began to enter our universe in the early 41st century, slaughtering or corrupting everything they touched. First one portal was found, then more appeared during the Judgment War, until it seemed as if they were swarming across the entire galaxy. When the Solar Alliance launched their desperate attack into this alternate dimension, they found a universe of darkness, almost entirely devoid of light, where the normal laws of physics held little sway and matter and energy were entirely at the whim of the entities within. The attack was successful at destroying the focal point of a massive galaxy-spanning network of crystalline planetoids that focused tremendous amounts of energy needed to pierce the dimensional barriers, and thus the main portals were sealed.
But the damage had been done, and human civilization was in ruins. Even worse, elements of the Great Enemy were still present in our universe, free to continue wreaking destruction, and possibly even build their own empires in the unknown reaches of space. Worlds that had felt their presence, no matter how briefly, were forever tainted, a seeping poison that would continue to grow on its own. In time, the local flora and fauna would still be corrupted, even without an active presence, and new portals might open to allow full-fledged demons through once again, centuries after the threat was presumed defeated.
Attempts at negotiation have always failed. What little communication has been had usually consists of threats or taunts on the part of the Great Enemy, or other attempts to break the resolve of any human defenders. Any hints of their ultimate motivation have only spoken to destruction and death being a fundamental part of their existence, an insatiable need as deeply-rooted as breathing. They are, quite simply, evil incarnate.
Unfortunately for the inhabitants of our universe, there seems to be no way to truly stop the Great Enemy. Their hordes appear to be endless, or at least so vast as to be essentially infinite, and aside from the lingering corruption they leave wherever they go, their most powerful members are not truly destroyed when slain in our universe, merely banished back to theirs for a time, where their essences can regather their strength and return to attack again. Any victory against them seems to merely be a delay against the ultimate destruction of the universe, and lasting success likely will never happen without some sort of divine aid.