Sin War

The Sin War was an armed conflict between the forces of the Black Empire, led by the insidious demon princes and the Carnal Armies, led by the Aesir via the mortal champion Ram'Sed.

It led to the overthrow of the Black Empire by the powers of the Alliance of Common Peoples, who shattered the demonic citadels of power and ended millenia of Hell on Earth.

Prelude
The Black Empire was Middle-Earth under the thrall of the evil demon princes. Seven in number, each prince had a citadel (a seat of operations in their capital city), a temple within the city, and a vast legion of demons at their disposal, all crafted using local - and very convenient - slave labour. When they landed on Earth, the demons did not expect any forms of life to have existed, so they were pleasantly surprised when they realised they could telepathically enslave the native populations by bending them to their dark wills. Thus, the Empire was built.

Middle-Earth grew more and more corrupted as the years passed, and the Aesir decided to intervene personally, for if they didn't, the final light of the Universe would blip out of existence. Sending in his Avatar, archangel Varian set out to raise an army among the mortal men of the planet. By rallying the Undertroll, Ram'Sed, and helping him to become chieftain of his tribe, an interracial Alliances of Common Peoples was able to be formed, with all the trolls linking with each other in a cohesive attempt to destroy the Black Empire. They became known as the Carnal Forces after other races joined the alliance.

The campaign
The war commenced soon after the year -25,600, when the Winter Uprising took place in the city of Pergamon. The Alliance set up an autonomous region under the city, having broken free of Belphegor's telepathic slavelink, a psychiatric bond that prevented one's thoughts from being free as long as they were under the link. Rallying forces, the march on Pergamon began the next morning, and the combined might of Ram'Sed and Avatar took the city. This was followed by victories in Xagresh and finally Belphatar, where the horrible Belphegor himself lay in wait. He tortured Ram'Sed horrifically, but Varian shattered his telepathic influence and bound him in a halo of light.

The same pattern took place in Asmodee and Belzeebub. Knowing they weren't strong enough to take on Lucithel yet, the Alliance gathered in strength and pace, before targeting Leviathan. Defeating him granted them access to the seas, and Isodael, via his Avatar, was able to create rafts for them to sail across. He was a former Scholari, a servant of the inventor-god Meinolf, thus his experience came in handy. Then, by sea, Sathanus was de-crowned, though it took a while for him to be subdued because of his furious attempts to escape custody (and thus, two halos were needed to end him, bringing him to the point of destruction).

Infiltrating Mammon's territory was a little bit harder, as the Lord of Greed was unfortunately well-prepared for them. Having heard about the fall of the other princes, he quickly snapped up their empires and channeled their energies into himself. His empire was also very expansive, full of nasty traps and jails. More losses were suffered here, thus, than at any point during the Sin War campaign, as Mammon did his very best to psychologically, physically, spiritually and magically subvert the opposition. He was undone by having his telepathy lines severed by holy inteferences, as light magic crippled their connection. The capital city, Avaricia, finally fell, and his temple was raided...but Mammon was not there. With the last fibres of energy left, he had teleported himself to the distant city of Mamaton, hoping to rule in solitude for the rest of his life. Nope. By the time Ram'Sed and the Aesir avatars reached him, he was sapped of power, and thus easily bound into a Halo (given that he'd spent his magic trying to teleport away).

Finally, Lucithel was attacked. The Carnal Armies were slaughtered by his demonic magic, and his dark possessions caused many a carnal to turn on his brother/sister and start attacking their own side. Thus, all the Archangels needed to form Avatars and enter Middle-Earth. This was a severe risk given that a) they could all die at once, leaving Lucithel victorious - they were more vulnerable in avatar form than true form, and b) Sargamon could take advantage of the situation and use it to steal their physical bodies while their souls where in the avatars. Pooling their holy magic, the Aesir were able to create a chain of light bombs that shattered Lucithel's citadel and toppled his Spire of Darkness. Lucithel, thinking it was about time to release his power, allowed the Spire to fall. When it smashed open, it imbued him with monstrous demonic energy, causing him to grow to behemoth size and totally obliterate the Carnal Forces. He was unstoppable in the physical form at this point, as proven when many a sword, hammer, arrow, axe or even boot were stuck in his body, only for them to fall off. Roaring with laughter, Lucithel prepared to bring all of Middle-Earth under his thrall, and reign as undisputed King of the Demons. He was about to attack the Aesir when Ram'Sed intervened.

Regael granted him Guardian's Fervour, and with it, he whipped himself into a battle frenzy, slashing at Lucithel and keeping him distracted long enough for the Aesir to gather the last fibres of magic their souls would possibly permit them. Stretched to the limit, they fused their powers and released it in a devastating chain of arcane blasts, sending piles and piles of halos onto Lucithel. At first, he threw them off easily, but, progressively distracted by Ram'Sed (whom he thought he could deal with easily) was worn down. Finally, with a swipe, Lucithel killed Ram'Sed, only for Duterius and Mezarel to donate life force towards the attack by siphoning the powers of their avatars to release one last chain of halos. Sapped of energy, the two were left with lethal exhaustion, but the battle was over - Lucithel, unable to suffer the successive piles of halos on his body, exploded into a million pieces, with dark magic left scattered across Middle-Earth. Anariel saw her brothers on the brink, but Ram'Sed's blood cried out to her, and she pitied him first. Granting him a Second Chance, Anariel blessed Ram'Sed, reviving him. She then healed Duterius and Mezarel before they grew even more tired. The Black Empire was over.

Clean-up process
After Lucithel was slain, the broken and weary Alliance began the long process of de-demonisation and clean-up. The Aesir imbued the Carnals with temporary power that allowed them to clean up Lucithel's remains and the magic they left behind. The halos binding the other demon princes were fading away at this point, as the demons were slowing twisting its light magic into shadow. Gravely concerned, the Aesir imprisoned the six living demon princes in subterreanean plates, sealing off their evil for good.

Trivia

 * The name 'Sin War' is borrowed from Diablo, and also involves angels (Aesir) and demons (Black Empire) fighting for control over the mortal men of the world (the Nephalem, who live in Sactuary - the equivalent of Middle-Earth in the AoC universe).