I distinctly remember a line, either said by Moren (the Crystarium librarian) or one of the children from the crystarium, or it might have been lyna or another npc.
It sounded like a verse in a song, and it talked about how the entity known as the "Warrior of Darkness" is a neutral force that is neither hero nor villain, and simply appears when there is an imbalance in the light

As we know, when we talk about the warrior of darkness, we refer to Ardbert and his companions, however that is simply because that's the first encounter with them in the source.
In the first, ardbert and his companions are known as the warriors of light.

I have had a hard time finding this specific cutscene that talks about the warrior of darkness, in the context of the First, and also about what it was before the scions appeared in the first.