I think this gives some clues to the story (from fansite)
Divinity II brings you back once more to Rivellon, a timeless world of awe and magic, torn and scarred by successions of apocalyptic wars. What the peace-loving people of the land did to deserve such terrible calamity, not even the gods can tell, but as any of their warlords will snidely say, the time to wonder and question quickly vanishes when the Lord of Chaos is slaughtering your kin, burning your cities and sowing the onetime fertile soil with salt. Twice this incarnation of evil burst forth from its hellish dimension to wade in valiant blood against the blackened sky, and twice Rivellon's defenders would not back down despite appalling losses. Still, victory brought no relief, for they knew that some day, the demon would be back.
And back he'd be, sooner than anyone could have anticipated, because, as is so often the case, evil festered from within: humans, loyal to the Damned Hordes, sought not only to ensure the return of their dark master, but to give his demonic form human semblance, so that rather than to destroy Rivellon, he would come to rule her. This group, known as the Black Ring, were close to achieving their goal and would doubtlessly have succeeded had it not been for both the tireless vigilance of the wizard Zandalor and the startling fate of an unsung adventurer. The stranger, guided as he was by the benevolent mage, exposed the Black Ring's sinister schemes and when the time came willingly underwent a daring ritual that infused this mortal champion with the powers of the gods, and so the Divine was born.
Leaving but grim corpses in his wake, the avatar of light followed the enemy to its stronghold, nestled deep beneath the wastes of Yuthul Gor, where he stalked and killed every Black Ring elder until finally he and their diabolic leader, the Demon of Lies, stood snout to face. The fiend smiled and told the Divine he was too late: the transfer was complete and the Lord of Chaos would walk again. Indeed, behind him, on a large altar lay a newborn infant, a shell of innocence around a soul of utter corruption. The Divine's sword saw the Demon dead, but despite being able to put an end to the vast plague that had almost brought Rivellon to her knees, he could not bring himself to kill the child.
He named him Damian and for years the Damned One, unaware of the terrible forces that brimmed beneath his boyish exterior, grew up under the Divine's tutelage. That was until he met Ygerna. Sent to seduce Damian by her father, the Black Ring Necromancer Kalin, she befriended the young man, who was instantly infatuated with her. Not only did she return his affections, but also his long slumbering powers. They practiced innocent spells at first, but later on more sinister magic, rarer incantations, and, most dangerous of all, they unravelled forbidden knowledge. For some time, Damian's Divine foster parent was blissfully oblivious of Ygerna's ominous influence on his son, until evidence connected her to Kalin, whom he recently executed. When questioned she confessed that she supported his rotten stratagems, the most important of these being the renascence of Damian's dark, dormant powers.
After hearing such hideous testimony, the Divine had no choice but to execute Ygerna in turn: the Black Ring never enjoys clemency, whatever the circumstances. Under the eyes of the wise, but worry-plagued Zandalor and an assortment of elite Paladins, the Divine's sword severed Ygerna's head from her body, and at that prophetic moment, while Ygerna's blood was still claiming more territory on the floor, Damian entered and gave voice to a spell that utterly stunned even the mighty wizard: the spell of Soul Forging. Before anyone could react, Damian turned back and seemingly disappeared. The Divine knew his son would from then on be his greatest foe and understood that Damian had already realised a great deal of his black potential: he who can Soul Forge, is a stupendous adversary indeed. The Damned One walked again.
As the Divine and Damian gathered their armies, Zandalor contemplated the repercussions of Damian's acts. A Soul Forge is an exceptional enough event in its own right; a Soul Forge with a soul as it dwells amidst the few fragments of time between life and death, is unprecedented. Uncertain of the consequences for either him or Damian, Zandalor entrusts Ygerna's body to the care of Deodatus the Embalmer, forgoing the usual ritual burning of Black Ring corpses.
Within days of these events, the Black Ring and Paladins clashed. Damian though, had eye for the Divine only: he would show him the same kindness he had shown Ygerna. What he did not know, is that the Divine was ready for him. He would lure his son to a recently discovered Rift Temple and, if all went according to plan, banish him to another dimension. Blinded as he was by his all-consuming wrath, Damian didn't realise he was being drawn into a trap and soon he was locked away in shadow haunted Nemesis. The Divine returned to Rivellon, glad that the threat his foster son posed was eliminated, yet strangely mournful because he realised that despite the evil that had taken hold of him, Damian's spur-of-the-moment Soul Forge was essentially an act of love.
The Damned One however, made the best of his situation in Nemesis. He bode his time, grew in stature and power, until he did what most thought could not be done: he broke free from his prison dimension and initially overran the surprised Rivellonian forces. His thoughts were still wholly focused on one thing: to destroy the Divine and so revenge Ygerna. The war changed the face of Rivellon: for years it raged and one catastrophic event follows the other. Where once there were mountains are flat, scorched plains and picturesque farmlands were pushed up and turned into jagged cliffs. Nevertheless mankind faced its infernal foes with remarkable courage and tenaciousness. A decisive reason for their stubborn optimism was the new forged alliance between the Divine and the rare, but immensely powerful Dragon Knights, the last and elusive proponents of dragon magic in the demon-swept realms.
Throughout the climactic battle, the scales of victory could have tipped either way. But then the unthinkable happened: one of the Dragon Knights betrayed and slew the unsuspecting Divine. During the confusion that ensued, the Paladins started to fight dragon and demon alike. It was Zandalor who rallied the troops and narrowly avoided disaster. Damian, who had already lost much of his forces and had seen his revenge materialised, ordered his army to abandon the battlefield. His dominion over Rivellon could wait. And besides, he had other things on his mind.
After the bitter stalemate that resulted, both sides took the time to lick their wounds and mankind prepared for yet another war. When this war did not take place after a year and not even after a decade, the good people of Rivellon were convinced Damian no longer posed a significant threat and started to relax and rebuild their lives. Now, as half a century has passed, the Damned One still has a more than frightful reputation, but is regarded as a distant threat at best. They could not have been more wrong...
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