Also yes that title is a Xenoblade reference.
With the end of Sen no Kiseki III, we are left with many lingering questions, and I think I’ve come up with something that answers a lot of them at once. One focal point of the ending is the sublimation of Millium’s soul into the Sword of Origin, an outcome that Alberich planned for her line of homunculi since their very conception.
Yes, it slew the Nameless One, but does it really serve no purpose now? Well, a lingering question that Sen III left us with is whether or not the Great One will even be able to split into its components of Blaze and Earth. If the Sword of Origin is able to cut through something divine like a Holy Beast, perhaps it will also be able to cut through the Great One, literally splitting it into its two components.
But there’s a problem with this, the Sword of Origin has only been seen to be wielded by Deus-Excellions, and reforming the Great One will require removing the power source from them, rendering them inoperable. Yet we know that Alberich has been tinkering in soldat technology for a long time, with the initial soldat designs being his to begin with. Perhaps the goal behind this all along was to create an artificial Deus-Excellion.
There’s still a problem though, the artificial D-E will still be lacking a power source. Well, in Chapter 3 of Sen no Kiseki III we see that by absorbing enough spirit energy, the Aion-type alpha II was able to temporarily replicate the “miracles” that it previously required power from the Azure Demiourgos to do. This same principle could be used to power an artificial Deus-Excellion.
So, Alberich’s machinations add up to creating an artificial Deus-Excellion powered by artificial means to wield a sword crafted from the soul of a homunculus to split a divine being in two. It’s pretty crazy sounding but it makes a surprising amount of sense.
Lastly, this also answers the question of why the three sides are working together. Alberich wants the Earth treasure, Ouroboros wants the Blaze treasure, and Osborne just wants the curse gone. All of these align with what is written above.