As she does this, Rivia's Barrier transforms into a magic circle of magical ad psychic energy surrounding the God-Essence. Within that circle she casts Soul Binding, a force to bind not just souls but essences. This binding will hold so long as Cas stays strong against it. His will, his soul now infused with Rivia's, will power the binding and hold the circle.
What she did however, allowed him to reassert what control he could manage over his altered being. It felt foreign, and cold, but brimming with more power than he'd considered himself capable, with the recesses of his mind whispering secrets of dark and ancient magic... One of which was accepted. A shiver echoed across both his, and Rivia's mind, as the secret was gleaned, but that was all.
After the merging was over, he looked at her and said softly, "... you shouldn't have done that for me. You don't know what might happen."
Rivia then faded from his mindscape and awoke back to where she and Cas were standing. The time was a mere 5 minutes from when they started, though it seemed longer. Her Fairies broke the circle and gathered around her, helping her down to the ground to rest. Once she was seated, they disappeared, allowing her a chance to rest her powers.
Casimir looked down at Rivia, both with thoughts of gratitude, and sadness. He really had no way to repay her for all she'd done for him. Hopefully he would at least be able to do what she'd asked him to..
Noticing that they hadn't moved, and most likely wouldn't for some time, he then sat down beside her. He was silent, content to watch the wind move through the fields of grain, then asked, "so... what did Xaric say about your family?"
Turning to him, "What about you?"
If she was alive, then Volkov just took a drop in the priority list.
When she asked about his however, he frowned. "No family to speak of..."
Rivia paused when Cas spoke of nit having a family, then smiled, "That's not as bad as it seems. You are now the one to begin a whole new lineage, your descendants will one day speak fondly 8f you."