Well technically speaking the Blight is a form of ultimate power if you use it properly.
Like Coryphifish , he was immortal and unkillable , he did a small mistake though with his dragons ,but if he didn't ...
Then through the Blight you can mindcontrol everyone infected.
So you can't be killed , you can mindcontrol all the blighted peons ,sure you look ugly and corrupted but so what?
I'm pretty sure their status as "Gods" wasn't going to last long , and the Evanuris knew it .
They had Fen Harel saying they weren't gods and freeing slaves.
The Forgotten Ones saying they weren't gods and busy getting more powerful.
There's the sinner showing everyone turning into a dragon was nothing divine or special.
They killed Mythal , sending the message well they could die.
And if the temple of Mythal wasn't propaganda , the other Evanuris were mistreating their own people.
Now my point of view is most of them probably deserved to be stoned to death , but I doubt they felt the same way.
There was this theory that Andruil created armor out of red lyrium, it seems like a reasonable speculation seeing that Samson had a red lyrium armor which made him several times more powerful...
One day Andruil grew tired of hunting mortal men and beasts. She began stalking The Forgotten Ones, wicked things that thrive in the abyss. Yet even a god should not linger there, and each time she entered the Void, Andruil suffered longer and longer periods of madness after returning.
Andruil put on armor made of the Void, and all forgot her true face. She made weapons of darkness, and plague ate her lands. She howled things meant to be forgotten, and the other gods became fearful Andruil would hunt them in turn. So Mythal spread rumors of a monstrous creature and took the form of a great serpent, waiting for Andruil at the base of a mountain.
Madness, "forgot her true face", darkness, plagues.
All these things hinted at red lyrium since it deteriorates the mind, causes disfiguration and could be made into armor and weapons (Samson).
Now, if the Evanuris mined red lyrium out of titan corpses it means they didn't unleash the blight, like FrankW said in one of his posts - it seems like corrupted lyrium isn't just blighted lyrium, the essence of lyrium makes the taint act differently. A non-mage consuming red lyrium becomes more powerful, much quicker but also loses their mind faster. Lyrium sings, the blight sings, their songs become one, creating another thing entirely.
This makes me think that red lyrium is a new thing, Solas doesn't seem to know what it is, at the Temple of Sacred Ashes he said that the magic might have drawn the lyrium and corrupted it, we know that isn't true and that red lyrium is blighted lyrium. Now, I'm not saying you can't corrupt lyrium with blood magic or something of the sort, but I'm pretty certain red lyrium isn't the source of the blight.
Another thing, if all of the Evanuris were just powerful mages, wearing a red lyrium armor would possibly mean the death of them seeing regular lyrium kills mages on contact (unless things worked differently in a world pre-Veil, something I doubt seeing I think the elves used the dwarves to mine lyrium safely).
Another plausible idea is that Andruil harnessed powers directly from the blight, the blight causes the mind to deteriorate and also disfigures people. Plagues and darkness also fits the blight better than the red lyrium and this part of the codex: "she howled things meant to be forgotten" makes me naturally think of the calling. Seeing Andruil was one of the Evanuris it is reasonable to think that she might have been able to cleanse herself from the taint given enough time.
While it does say the other gods were fearful Andruil would hunt them, it doesn't mean they condemn her actions, perhaps they were afraid of her newly gained powers and after Andruil's defeat hatched a plan to harness the power of the blight in a "safe" manner. For all we know their plan might have been to harvest dragon hearts (I have no evidence to support this, just the fact that dragons seems resistant to blight and cleansing runes are made from hearts) and use them to control the blight.
I also want to point out that "the Void" is the name of the place "The Empty Ones" thought to be the origin of the blight, it could be nothing but seeing that both elves and this cult spoke of that place I feel compelled to dive in.
I speculated before that the Void is another dimension, like the Fade, but I'm now uncertain, so then I thought, what if the Void is the area beneath the pillars of the earth? Beneath the titans. A place of "nothingness", utter darkness. I don't know why the blight would exist there, perhaps it has something to do with the tale of Elgar'nan throwing the "sun" to the Void, and that whatever that "sun" was got corrupted (from within) to become the origin of the blight. It could've been a powerful spirit, like a spirit of hope (if I search for symbolism) that got corrupted over time and was thrown by Elgar'nan to said place beneath the earth, perhaps said spirit created the first of the elves from the earth, out of curiosity or boredom, and eventually got jealous of their achievements and emotions. Either way, "sun" done goof and Elgar'nan imprisoned him so he won't risk his freedom.
This makes me think of this (heavily discussed) mural:
Why does these 7 orbs must be the Evanuris? Why can't they be titans, or the forgotten ones?
What if by killing a titan, the blight was unleashed? Not from the actual death of the titan, but the fact that there was no longer something stopping the blight from spreading. We know darkspawn don't set foot inside titans, what if that's because titans, like dragons, kill the blight? Or even that the song of the pure lyrium repels it so it scares of the darkspawn?
All these layers that Solas draws just make me think about our earth
What if the mural is more literal than we think? Sky, earth, titans, the Void...
Several points that I have no idea where to put in my (massive) post:
1. The taint can be cleansed with blood magic, but red lyrium can be grown from bodies (like regular lyrium, if consumed my mages through extended periods of time, like Corypheus).
2. Red lyrium isn't as "angry" in cold climates.
3. Fenris can pass through physical objects because of his lyrium tattoos which allow him to tap into the Fade while still giving him magic resistance.
4. Red lyrium is anti-magic and an opposite force to regular lyrium.
5. After writing the word lyrium so many times it starts looking like gibberish.
By the way, behind that orb of... eyes.. there is definitely a silhouette of a city, it's super intentional.