Although our records are fragmentary at best due to time and the ravages of the conflict itself, we know the Judgment War was the worst conflict in human history, vastly outpacing any other war in length, loss of life, and destruction.
In the 41st century of the old calendar, the Solar Alliance was believed to have numbered over two thousand worlds like ours within its area of control. There are no surviving records to indicate if it was the sole human government, or if there were others beyond its borders. Terranis and its parent star system, Delta Gamma 421, were on the edge of known space. Aside from being able to travel between worlds as easily as we might travel from one town to another, they possessed scientific and technological marvels far beyond anything we can conceive of today, even with what fragments of that technology which we still possess.
Into this seeming paradise came the Great Enemy, emerging from some sort of spatial rift allowing passage between their universe and ours. The early stages of the war are lost to us, but it spread like wildfire throughout the domain of humanity. For nearly 300 Terran years (approx. 360 years local), the Solar Alliance and the Great Enemy battled each other in conflicts of apocalyptic scale. Some scattered fragments mention entire planets being turned into warzones, or space-borne naval battles with hundreds of ships on both sides participating. There is mention of worlds being blasted into little more than floating rubble, and even implications that, on several occasions, the Solar Alliance triggered the destruction of whole suns in the hopes of defeating the Great Enemy. Given the sun was as much a holy symbol to humanity in those days as it is now, one cannot help but shudder at the level of desperation involved in such a decision.
As the conflict ground on, it seemed humanity was clearly losing, despite its best efforts. World after world was destroyed or overrun by the Great Enemy, and even our own technology was turned against us as the demons found ways of corrupting it to their use. In time, mutated creatures and even twisted humans were used, as we have experienced here on Terranis in the centuries since the Fall. What was once a domain of trillions of humans rapidly dwindled into billions, and then perhaps even millions.
I could find no reference to the state of humanity by the end of the conflict, beyond mention that the vast majority of worlds inhabited before the war had been lost or destroyed. Indeed, the only reason the war ended at all was because of a last desperate strike by the Solar Alliance into the dark dimension of the Great Enemy that sealed the portals between worlds, though almost none returned from that mission. This halted the flow of reinforcements, but those demonic forces still in our universe were able to continue their rampage. In time, one of these groups came to Terranis, bringing about the Fall.