Battle of the Darkness

The Battle of the Darkness, otherwise known as the Siege of Um'taran, was the final, massive battle of the First Wars, and undoubtedly the largest and longest confrontation between any armies throughout all of history.

Battle
After the devastating Battle of Zakkasil, the world looked to be on the brink of destruction. The Goddess of Hope, Thulnos, decided to indirectly intervene, sending a host of Celestials to aid the Eldar, who responded by gathering all that remained of their strength and rallying the Men to their cause as well. This combined Host moved north, crushing a Horde of Keth at Vangesh.

In response to this new and terrifying threat, the Flesh-Angels emptied the deepest pits of their fortress and called on Keth from across the continent and beyond, every single one of their twisted creatures issuing forth in an army beyond comprehension, so large that even Jhayita, let alone Drangesh, could not contain its might. Dragons, Trolls, Keth and even the dread Kasijaks of the Void all assailed the mighty host of good.

But sheer force of numbers was not enough. Millions of the enemy were incinerated by the otherworldly powers of the Celestials, and the Eldar cut down the Keth in their thousands. The Keth withered away, obliterated to the fullest extent of the word. The Dragons fell burning from the skies. The Kasijaks were all banished and imprisoned.

Now, the Flesh-Angels, who had not come out at all, withdrew into their impregnable fortress with their remaining servants to resist a siege of epic proportions.

The Hundred-Year Siege
The effects of this siege cannot be comprehended. The very land was uplifted, the earth rent and broken. The siege lasted for a century all told, meaning that the men all perishing of old age. This left a steadily diminishing host of Eldar that failed to breach the iron curtain walls. It was then that the Flesh-Angels were forced to issue forth (having finally begun to starve), wielding their obscene weapons with a fleet of the largest Dragons behind them. This final assault took even the celestials by surprise, and they were knocked back. The Eldar were almost destroyed, before the Flesh-Angels were overcome by the Celestials and exterminated. The Dragons were killed, save for a thousand that fled from the field. The Celestials delved deep into the caverns, razing the great bastions to the ground. But they did not dig deep enough. Deeper still lay the slumbering Dark God, who remained in state by the three surviving Flesh-Angels and the Accursed.

Aftermath
The impossibly vast Keth hordes were almost totally destroyed. Casualties had been almost disastrous for the Eldar, who ultimately failed to recover. Mankind, having recovered quickly during the century or more spanning the battle, sensed weakness in their ally and subsequently sparked the Eldar-Men War. Thulnos, Goddess of Hope, was punished by The One once her plans had been discovered by Him, and He said that all action within the realms of any world within the Void was therefore banned completely. The geological damage of the conflict had been tremendous as well, although much of Um'taran itself remained intact. And, worst of all, the Dark God had not been destroyed...