There's some ancient/mysterious weapon/being/thing that lets you kill Ascians easily, or at least easily enough. Have to remember that killing Ascians is still messy currently in the story, there was the whole spear thing from Moenbryda but that doesn't work so well without someone dying, and knowing we're going up against even more Ascians, including the father of the whole Empire, it makes sense we need to be prepared.
As to how that ties to the Empire story bit, I'd say it works out fairly well. Why does Varis act like a dickhead? Well he said why in the last story, he wants to Rejoin everything so humans become powerful enough to kick the Ascians asses. That's really his obsession and why he goes along with all the garbage, because bringing about the Rejoining would MAYBE let us win in the end, at a severe cost. So how do the Scions work against that? Well, if we remove the Ascians without needing to sacrifice everyone, that sounds pretty good yeah? If the Ascians are removed, maybe we can convince Varis to stop the war. At the very least if we can kill Solus, we can prove there is another way, and we can convince Varis to not kill most people to get there. If that involves killing Gods and what not, well, why not.
And so we set forth to the First, to find a way to slay Ascians without bringing about the Rejoining, and to actually defeat whatever gives us that power of Light, we need to become the Warrior of Darkness. It's a very standard "power up arc" anime thing. And that's also why the others are left to "hold the line", as best girl so eloquently says during the trailer. How Minfillia fits into that, I'd assume we need her to travel to the First, simply enough.
