I believe that Radahn held the stars in place when Ranni began fearing her fate. She never wanted to become a puppet to the two fingers, and Radahn knowing this attempted to hold off her destiny for as long as possible. Though he likely knew that it would be an inevitability. The only other two living empyreans were cursed afterall.
There was no way that Miquella or Malenia would be able to take Marika’s place, the two fingers weren’t even the slightest bit interested in either of them. Malenia was already claimed by the God of Rot and Miquella was perpetually in a state of adolescence. Ranni seems to be the only one who was ever actively bothered by her fingers and I believe this is because they had an intense desire for her to be the next god, assuming Miquella and Malenia as lost causes due to their curse.
I think this is when Miquella came to Radahn with a proposition. He will be seeking godhood. Even if the two fingers had no interest in him, even if the world thought him too small and frail to ascend, he was going to find a way, no matter the consequences. He was determined to become the next God and lead the world into an era of abundance and peace. To fix the mistakes of the past.
He only had one simple request.
“Promise me, you’ll be my consort.”
If Miquella were to ascend then Ranni would be free of her duty. She’d be able to live in peace knowing that she’d never have to be controlled by another's will. She wouldn’t have to become a god, she wouldn’t have to succeed queen Marika. And to top it all off, Radahn would get to play the role of his hero Godfrey. Ushering in a new era alongside a new god.
It was perfect.
That was, until the night of the black knives. The night the world was thrown into chaos. The night that the first demigods fell. And the night that he lost his sister.
Radahn likely wouldn’t have known about Ranni’s plot and would have thought her dead alongside the other victims of the night. Ranni was gone, and with her, so too was Radahns need for Miquella’s age.
I believe Radahn, fueled by grief, rage and the mad taint of his greatrune, denounced Miquella and struck out on his own as a warrior of the shattering. He didn’t need peace, he didn’t need Miquella. All he wanted was to fight. All he wanted now was endless war.
Maybe that would be enough to dull the pain of his loss? Not only the loss of his sibling and father, but also the loss of his purpose.
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Now picture Miquella. Making the final preparations to begin his ascension. Only there’s one problem. His consort has abandoned him at the very last moment. As it turns out, it seems Radahn never really cared for Miquella or his visions for a new age, he was only really in it for his own gain.
Imagine how devastated Miquella would be. Knowing that yet another one of his plans had fallen apart at the finish line. That the person he trusted to rule alongside him didn’t even hesitate to abandon him when it was no longer convenient for him.
His ascension to godhood would never happen. He would never be able to heal his sister. He would never be able to bring forth an age of compassion. He would never be able to right the wrongs of the past.
He had failed again.
Now imagine how furious this would make his people. How angry his followers and his sister would be to see Miquella rejected at such a pivotal moment. To see him mourning a consortship and an age that would never come to pass.
Is there really any wonder that they chose to march towards Caelid and deliver the promised consort by force?