I have formed an idea. It's more of a hunch really, all this dragon talk has been very stimulating. I guess the most I can hope for is to ignite a spark of theorycrafting in someone else's mind as I have only been trying to piece this together for the last 2 hours (although the general idea has been bugging me for a greater while now).
Some excerpts from the DA wiki, my other muse: (this thread is still muse no.1)
Long ago, when time itself was young, the only things in existence were the sun and the land. The sun, curious about the land, bowed his head close to her body, and Elgar'nan was born in the place where they touched.
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He lifted himself into the sky and wrestled the sun, determined to defeat him. They fought for an eternity, and eventually the sun grew weak, while Elgar'nan's rage was unabated. Eventually Elgar'nan threw the sun down from the sky and buried him in a deep abyss created by the land's sorrow.
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The Dalish believe that when Elgar'nan buried the sun in the abyss, some of its heat remained in the Earth and gave rise to hot springs in the Dales. They call the springs the Pools of the Sun and treat them as sacred. They also believe that the sun's lifeblood spilled in the battle with his son became the stars.
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Some legends paint Andruil as a daughter of Elgar'nan and Mythal while others suggest she is a child of the earth.
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Her love for challenging hunts drove her to looking for prey among The Forgotten Ones in the Void, something that ultimately drove her mad.
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According to ancient elven writings Andruil is creator of some great and terrible weapon, a throwing spear of unbearable heat, crafted from radiance of stars.
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The blood of Great dragons can confer enormous power. According to Aurelian Titus in Dragon Age: Until We Sleep, the Dreamers of ancient times commanded The Fade. The last Tevinter magisters were close to reclaiming this power, and "All they needed was the blood of the Great dragons."
Ahem. Let's do this! *cracks joints*
Elgar'nan was originally a child of the earth, just like Andruil. To gain power, Elgar'nan took it from the dragons who were still ruling the world back then (=the power of the Creators came from the dragons). Something went down and he wanted/had to overthrow the dragons - he cast them into the abyss/void. Taking inspiration from RawToast's post, I'd now postulate that the dragons were still held in high regard and hunting them was forbidden. Along came Andruil who felt the need for a challenging hunt so she went dragon huntin'. In the void. Note how the Dalish legends say that sun's (dragon's) lifeblood became the stars. Guess who crafted weapons from the "radiance of stars". That's right, Andruil, who had been hunting in the void. So Andruil created a weapon using dragon blood, which is said to confer enormous power.
Could Andruil's weapon have been red lyrium? The pools of tears the earth cried sound like lyrium and the abyss was created by the earth's sorrow after all. Then [blank] and [blank] which means [blank] is [blank].
As you can see, there are countless problems with this. First, this sort of implies the Forgotten Ones are dragons and I don't want to believe it. Also, no explanation for the Taint. Maybe it'll come to me in a dream.