Bellefleur wants to do more than end this history: she wants to end all histories. She wants to bring about the final victory of the Emperor-Wolf, and drop the curtain on the universe.
How does she plan to do that (and what does Bastian have to do with it)?
To understand Bellefleur's plan, you have to understand (as she does) the actual purpose of the universe as conceived by the Glory itself. (And I'm going to have to be a little evasive here, because Emily hasn't worked out everything going on with this stuff yet.) The Glory has no will, per se: but the exhalation of the Glory at the dawn of the Anthelion history created the entire architecture of the universe as we know it, as if it were a natural and inevitable consequence of the Glory's emanation.
Into that universe, the Glory projected - well, it's called a lot of things across many cultures. A son, let's just say. Bastian understands it to be Lucifer: the greatest of all the Glory's creations, the one most like the Glory itself, but a being of dynamism and will and preferences, set to rule over everything in existence. Another way to think of it might be to just say that the Glory - which is in completeness and totality in a way that may preclude it from holding anything as prosaic as an opinion - created a projection of itself that was separate from itself, metaphysically incomplete in a way that would allow that projection to distinguish between itself and the universe around it.
And then the Glory asked his son - this projection, this emanation - a question.
Bastian doesn't know what the question was. But only Lucifer - as he calls this creation - can answer it. And Lucifer can't, presently, answer the question, because Lucifer no longer exists.
In the moment after his creation - right after hearing the question - the son of the Glory discovered darkness inside of himself, and expelled it to the depths of Mithra, the gravitational nothing at the bottom of creation. That darkness embodied itself as the Emperor-Wolf, but it's hard to say that the Emperor-Wolf is anything. It is nothing: it is the void: it is the negative image. It is the nihilistic howl of unmaking. And for the rest of the Anthelion History, the Emperor-Wolf planned its revenge against (as esoteric scholars in the world of Heretic call what was left of that son of the Glory then) the Sun-In-Glory, for presuming to define and create his perfect Self as one without a shadow.
The Emperor-Wolf created the Rending - an annihilating force of hatred and destruction - and hurled it out of Mithra into the heart of the Sun-In-Glory, and killed him.
And then a new history turned over, and the Rending did not kill the Sun-in-Glory, and this really pisses Bellefleur off: because the Emperor-Wolf had won. The Glory (if it is the Glory who determines the fission of a new history) had cheated. In this new history - the Pridian History - the Rending had shattered the Sun-In-Glory, but not fatally. What was left was
The Crown (the Sun-In-Rags, in hospice now under the care of the Madrugad, still in possession of the Voice of God)
The Mantle (the Pyre-Hawk, the burning beauty and joy of Lucifer)
The Body (Mother White, the strength and creative power of Lucifer, now undead and starving)
and the Blood, who is always mortal, and is...whatever is left of the Devil when you strip the rest of it away. The Blood is who the Devil is. What sort of person he is. Bastian is the Blood of his age. Christ was the Blood before him. There have been others.
What does all of this have to do with Bellefleur's plan?
Well, the universe can't end until the son of the Glory answers the Glory's question. That is - from the perspective of someone like Bellefleur, who is looking at everything on the grandest possible scale - the point of the universe. It is there to be regarded by Lucifer and inform the answer to the question, whatever the question might be. So to end the universe, first you have to put the Devil back together.
And that includes the Emperor-Wolf and the Rending, too! The part that Lucifer expelled from himself, and the part of that which the Emperor-Wolf fashioned into a weapon to be his active agent in the universe (mirroring the relationship between the Glory and Lucifer himself, if you like). Only when all six are one being again can Bellefleur hope for the histories to stop changing over. This project is known by esoteric scholars in the world of Heretic as the Reunification of the Sun.
But if the Sun is to be Reunified, it has to be determined: under which part?
The Emperor-Wolf and the Sun-In-Rags both want to 'rule' over the reconstituted Son of the Glory. Mother White seems to want that, too, if we can interpret "she wants to eat the rest of them" as 'ruling them.' The Pyre-Hawk doesn't seem to care who comes out on top, but has not joined its power to any of the others yet (and once did make a friendly overture towards Bastian, which he accidentally rejected because he didn't realize what it was): and then there's the Rending, who just wants to destroy everything, and maybe go on a date with the Sunflower King, but definitely wouldn't let any harmonious conjunction of the constituent forces of the Sun happen on its watch, unless perhaps it would leave the Emperor-Wolf, its father, ascendant.
The Blood always starts out as, BY FAR, the weakest of all of the parts of the Sun. But it's also the only one that isn't trapped. The Emperor-Wolf is imprisoned on Candle Lake, the Rending fell in love like a dipshit and is kept pacified by the sacrifice of the Sunflower King, Mother White is pinned down mostly-dead by the Black Captain and the Madderblade, and the Sun-In-Rags will die if it leaves the citadel of the Madrugad. (The Pyre-Hawk is just vibing. It's not trying to win.) So Bastian is free to act in a way that none of the others can.
Bastian intends to complete the Reunification of the Sun under his banner, and answer the question the Glory asked himself, once he figures out what it is, and how to make an answer. Bellefleur also intended that...up to a point. She had a great plan that would see Bastian conquer Mother White, the Rending, the Pyre-Hawk and the Sun-In-Rags before confronting the Emperor-Wolf (a plan which, to Bastian's great unease, his is probably still following). But Bastian intends to defeat the Emperor-Wolf: while Bellefleur planned to betray him and sacrifice him to the Emperor-Wolf, securing the final victory of the Void.
Bastian's still gonna have to confront the Emperor-Wolf eventually. And Bellefleur surely still plans to turn up when he does.
As for why Bellefleur wants this, I can't really say much. Why his wife who he loves meant to stab him in the back at the precipice of his triumph is a question that keeps Bastian up at night. She's angry at the Glory for the state of the universe, sure: but wasn't the plan for Bastian to change the universe? She saw the Emperor-Wolf with her own eyes in a moment of despair while on a last, desperate pilgrimage for meaning, but did that enslave her to his will - or did she genuinely find purpose in his nihilism? She was launched on this course after petitioning the Queen-In-Chains, but does that make her quest madness, or inspiration?
What Bellefleur really wants, and if she can possibly be saved from wanting it, is a big question for the final act of the campaign, when we get there.
