My attempt at rationalizing the bonkers game that is Sen III, featuring the favorite word of the first two games: “Power”, and the favorite of the third game: “Curse”.
Towards the end of Sen III, we are introduced to the concept of “Steel”, also known as the “Great One”, a fusion of the Blaze and Earth Treasures. During her exposition, Roselia states that “The Blaze and the Earth remained locked in an endless struggle for power within the body of the Steel.” This is a continuation of the ancient battle between the treasures, where even after both civilizations made peace, the Gods refused to stop fighting until both of their energies had been drained, which resulted in the creation of the Steel.
We are then told that “The kin of the Blaze Treasure worked to split the Power contained within the Steel into seven pieces, while the kin of Earth created Seven Vessels to contain that Power. The body of the Great One was sealed in a higher plane, however, some of its essence (Curse) still remained.”
Now, we know that Ouroboros are after the treasures. So even if they manage to unleash the curse and gather the seven fragments of power, and release the body of the great one from a higher plane, how will they split the treasure back into its original forms, the Arc Rouge, and the Lost Zeum.
The slaying of the Holy Beast of Earth is counterproductive in this sense. In order to reform the Great One, you’d likely need the curse to be in one place. So was there perhaps another reason behind killing the beast?
The simplest way to split the Steel back into the treasures would be to end the fighting between the Curse and the Power. We know from Zoro-Agruga’s cursing of Testa-Rossa that a single fragment of the power stands no chance against the curse, but what about all seven?
Even at this point, the Power would likely be equal to the Curse, and so the battle would continue on forever as it would have. However, we know that a good weapon contributes substantially to a Deus-Excellion’s potential, so perhaps if six more souls were sacrificed, and six more weapons of Origin were created, the curse may be able to be defeated.
By this logic, was the purpose of starting the Great Twilight merely a means to establish a method of creating these swords, and ensuring their power? I wouldn’t trust Cedric with one though.