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Solas, better known as Fen'Harel, is an extremely powerful and ancient mage who is wrongly believed to be a god.
Long before the Dragon Age, Solas was among the very powerful mages who were wrongly titled as gods, Solas rebelled against the other tyrannical gods and freed their slaves, after the other "gods" killed one of their own named Mythal. Solas resolved to create the veil, separating the Fade and the physical world and banishing the other "gods".
However creating the veil also reduced his people to a shadow of their former selves which Solas resolved to reverse so he went into hibernation while his orb gained enough power. Once he awoke he was too weak to activate the orb and manages to arrange for a powerful being known as Corypheus to obtain it and unlock it, however Solas hadn't expected that Corypheus would both succeed in unlocking it and survive and in so doing Solas accidentally created a threat to the entire world and resolved to stop him by aiding an Inquisition that was created in order to stop him.
Solas joined the Inquisition to stop Corypheus and reclaim the orb. He helped in killing him but the orb was destroyed.
After the death of Corypheus, Solas left the Inquisition and resumed his own plans and Flemeth gave him her powers to help him. When the Inquisitor finds him two years later, Solas explains who he is and that he plans to destroy the veil and that while this would restore the elves to their former glory it would also cause untold chaos and possibly destroy the other races of Thedas, the Inquisitor vows to stop him either by redeeming him or killing him.