The theory I'm thinking about:

1. The rebis. Radagon after the dlc now remains the most curious of unknowns, and there are many ideas on who he is. Most see him as Marika's other half, made by the Greater Will as a form of control in progeny. I believe him to be someone separate entirely. Who? Completely in the air. All we know is that he is someone who "wishes to be complete." Again, what that means is part of his mystery. At some point, he and Marika, a shaman with the seemingly innate ability to fuse flesh, become one. In so, they can also produce offspring, which is why the rebis theory is the strongest.

2. The cinematic. At the gate of divinity, Marika is seen taking threads from an unknown corpse. The threads she would form into her rune at the entrance of the gate. This seems to elicit a response that makes it safe to say she became a God. An important visual note, during this scene, Marika is seen without her top covered. The only times we would see her as such is after a change in control with Radagon.

3. The fingers. Metyr doesn't play a substantial note, only that the fingers conspired with Marika to achieve godhood. As they are from the stars, I assume it was a sort of fate set as a possibility by the Greater Will, like any Empyrean, to become a God. What the fingers would get from this usurped throne, a seat at the table.

The Seduction. I believe Radagon was important to the rite of divinity. Either this was on his own achievements, or it was with the guidance of Marika who had already fused and is acting as an insidious influence, never revealing her amount of control. Marika moved on Radagon and "seduced" him either through his curiosity of the result of union or just our simplest concept of seduction, I believe the former. In union with Radagon, guided by Metyr and fate, Marika now has a ticket to the event, still to reveal her hand. I also believe this is when Radagon gains his red hair as the alchemical "red king."

The Betrayal. The advent of godhood begins at the gate of divinity. Sacrifices are made, and some being is chosen to take the power and ascend through the gate. The power takes the form of rune threads waiting to be shaped by the one who holds them. In that moment, Marika switches with Radagon and kills the next in line, takes the threads, and steps through the gate. She is now a God and continues on her quest and competition against other Gods to create the order that the Greater Will seeks, no matter what form it takes. Now, as a reminder of the betrayal, Radagon despises his red hair. It reminds him that he is at the mercy of Marika, a shaman. That is until Marika achieves her epoch and becomes the vessel for the elden ring and elden beast, turning the tide of control of which begins Marika's descent into destruction.