More about the Betrayer and the Thirteenth [Spoilers]

[MrThinker]

/snip

Yes, you are right, going over the (English) quest dialogue again, it really makes it hard to understand.

/snip

so that part of her english dialogue is pretty straightforward and matches what she says in the quest:

The task was difficult, but the plan simple. Souls predisposed to Light, fledgling heroes in the making. I would bring together Ardbert and his comrades and groom them for a higher purpose.
I would play the part of comrade and guide their course, all the while laboring in secret to pit stronger and stronger enemies against them, until finally, once they were Warriors of Light brave and true, I would reveal myself as a Warrior of Darkness and extinguish their radiance forever.

The confusing part is what she says right after:

It was not to be. I had misjudged Ardbert. His comrades before him had made their sacrifices in turn, and as their mirror I had grown stronger for it. Had he but embraced his role and struck me down, victory would have been mine...
But Ardbert, dear Ardbert, would not forsake his heart.
In the end, he chose mercy. He chose love, and I was undone.

To me this reads that in order for her to win, she first needed to be struck down by Ardbert (and thus 'by his sacrifice' he would become a chosen of Hydaelyn), and then somehow come back to life stronger to kill them (Zemus to Zeromus transformation). But because Ardbert refused, the ritual she used to empower herself with darkness failed.