I have a theory about who killed Mythal and why.
I guess that the one who killed Mythal must have been close to do the deed, since I doubt it was easy to kill her, and it would have been someone she would not suspect. And when I read the song about Falon`Din at the temple which said:
The people swore their lives to Falon`Din
Who mastered the dark that lies.
Whose shadows hunger
Whose faithful sing
Whose wings of death surround him
Thick as night.
Lethanavir, master-scryer, be our guide,
Through shapeless worlds and airless skies.
It mentions him having wings that is thick as night(which is to me another proof that he was a dragon like most of the creators, and the part about shapeless worlds and airless skies is obviously talking about the Beyond.
Then I thought that both the elvhen religion and chant of light say people pass through the beyond/fade when they die, but not a single time has a person who died been ever seen in the fade, which I do find strange since someone must have come across one if all who that can dream dies and pass through.
but there is a single place in the fade where no one except the ones who are either dead or has not said anything about, the Black City, I guess the Black City is the seat of power for Falon`Din and it is a place where only the dead can be.
Then I remembered that Solas mention that Falon`Din started a war once to gather more followers to him, and he did not surrender til he was beaten bloody at his temple bu his fellow creators.
I also remember the story about how close he and his brother Dirthamen is.
Then there is the Order Of Fiery Promise of the Seekers that believe the world must die in order to be reborn again, and a codex entry about them mentions that they have always existed among the seekers and many times has the order been purged of them, but for some reason they rise up again among the Seekers out of nowhere and no one knows why or how.
And when Flemeth absorbed Kerans Old God soul, it looked very similar to how she sent Mythals own soul through the Eluvian, Flemeth also said that as long as the song is playing, we dance(which I think is the song the of the Old gods she is talking about).
So this is my Theory:
Eventually Falon`Din started to believe the world needed to die to be rebron again and even made the Humans worship him and creators in different name so that he could use them later on, but Mythal and the others was against it, but Dirthamen could not fight his ow brother again like last time and he had been turned to his brothers side without the other creators knowing.
So when the creators and their followers had gathered to meet Falon`Din and his followers on the battlefield, but then Dirthamen stabbed Mythal in the back by using either somekind of magic weapon or power, and amidst the confusion, their followers started to fight against each other, and the creators started to fight among each others for both more power and because they did not know who to trust anymore.
And when that happened, Falon`Din used some dark power to put the other creators into a deep death-like sleep(and if he was a bigger ass and make more sense as to why Dirthamen would side with his brother even after knowing he wanted to destroy the world, he put Dirthamen into sleep aswell) and Buried them deep underground.
the thing that happened after was of course that the Elvhenan Empire fell, and with it gone, one of Falon`Dins biggest obstacles was gone.
And to make sure they would not rebuild their empire or regain their full power, he told his human followers to take the elvhen as slaves and destroy their knowledge and history.
After that, he made sure that the Tevinter Empire took over all of Thedas, adding great destruction and death in it´s rule(summoning as many demons as possible and blood sacrifices for power so that it would add to the destruction of the world), but he did also try to find Fen`Harel who had escaped him.
But the Humans did not kill or destroy as much as he had hoped for, they did show more restraint to complete destruction then he had thought, so he made new plan which would guaranty total destruction.
He told some priests in their dream to try to walk into the fade and become gods, they did do so, and walked into the Black City, but since it was a place for the dead only, it changed them. It infected them with a sickness of death, which we all know as the Taint.
With it, the world was truly plunged into destruction and death and Falon`Dins goal was at hand, but it was stopped when the Grey Wardens came(and I suspect Fen`Harel was the one who told them about the Joining, since it would make sense).
When the Blight failed, Falon`Din knew that it was Fen`Harel who had helped them defeat it, and so he put a curse on the sleeping creators that make the Darkspawn hear a song from them so that there will be another Blight, while he goes searching for Fen`Harel again.
But then the Tevinter Empire fell, by a woman who seemed to have the powers and Knowledge of Mythal, and that scared Falon`Din, that Mythal soul was still here, and he made sure that this woman with her soul did die, but the Soul traveled from host to host, until it came to Flemeth.
Now knowing that Mythal soul was still out there, Falon`Din knew he could not keep doing what head been for the last millennium, so he went into hiding, manipulating people to do what he wanted, and making sure that no Dragon would join Mythal, by making Humans wanting to hunt them to extinction, even creating a secret order with power, that shared his views and goals(The Promise). And of course stopping any chance for the Elvhen to rebuild again, even making the last of the priests he manipulated think he could become a God.
But so far his plans have all failed, and soon he has no choice but to go to open confrontation with Mythal.