Alright...we all know why we're here. Just a few notes:
This is a work in progress
I may not have 100% of my facts checked on various lore characters yet.
Feel free to suggest anything to add depth to the story. Part 1 is currently lacking. Planning a bit for Imperious to link in with Part 2.
Part 1: Adria
Adria had learned of the events that transpired in the high heavens and raced to find the fallen black soulstone. News of this reached the Angiris counsil as the citizens of <insert town/city> reported attacks from the underground Alchemy labs led by the treacherous witch. Imperious decided to chase Adria whilst the heroes dealt with the witches and experiments in the Alchemical laboratory.
As Adria scoured the country for the stone, Imperious planned a trap with which to lure her into a position that she could not escape from. By the time he had found her, the heroes had finished their task and came to assist Imperious. However he had not waited for them to appear and challenged Adria as they arrived. She had not expected the archangel to find her so soon but he had given her time enough to begin an incantation. In an attempt to dispose of them, she transported the heroes to the edge of the burning hells. There, she called upon one of Diablo's most feared terror demons to defend her. However, the demon did not aid Adria, instead it picked her up and threw her over the precipice.
As these events unfolded, an image of Malthael appeared with the black soulstone within his hands. Adria had merely been a distraction to keep the heroes from interfering with his plans. The stone had been easily captured by him during its fall from the heavens. The heroes would have to fight the crazed demon and prepare to bring the fight to Malthael.
Part 2: Malthael
The archangel had been distraught ever since the loss of the worldstone and after hearing the news that Diablo and the other lords of hell had been vanquished and trapped within the black soulstone, he believed that the scales had been tipped out of balance. In his mind, Wisdom was to find a balance between heaven and hell. Sanctuary had proved promising... but Tyrael had destroyed this dream and so, both angels and demons seemed to be flawed and a balance had to be maintained between the two. Thus he decided that the souls of the demons trapped in the black soulstone should be set free once again within the burning hells to bring balance to sanctuary.
The heroes had not fought any angel before and so, eager to redeem his honor for not being able to defend the high heavens from diablo's corruption, Imperious decided it was his time to fight. However Malthael had already made plans for Imperious. Using power from the black soulstone, he brought out the dark side of Imperious. The shame of his defeat started ripping Imperious's soul apart and so, the heroes sought to intervene. But Imperious fought them back and challenged them to prove themselves - to show them why they were able to defeat diablo and he was not. Whilst Imperious fought, Malthael started the incantations and the rituals required to return the souls from the stone...
Be greatful to hear your thoughts on the story so far!