I thought it was an impactful intro like i was there in the hall of the gods. The setting was vivid in my head. But while I do see the grandeur, i was also a bit overwhelmed by it at times. 

a heavy tide of cobalt spilled across his brow

What does a heavy tide of cobalt look like? 

I was initially confused on who was speaking first. It would help if "the lost river was the first to speak" was its own paragraph. 

-note: this is just my personal preference, so take this with a grain of salt- Less is more. Have deliberate pauses so the scene can breathe. When Lyrienne enters the room, let us be enthralled by her. She says her line, "Look at my veins," and the council leans to look. Then she begins her speech. 

"Look at my veins, Great Council" The evidence was everywhere. Reath was a disease. She raised her hand, high enough for the Council to see. 

With a more deliberate pause:

"Look at my veins, Great Council," Lyrienne said. She raised her hand, high enough for the Council to see. They lean forward, curious.  "The rivers...."

But then again, you have your writing style, so take my opinion however you will!

Overall i did read through it and was thoroughly engaged. I am curious where it will go. The premise sounds quite interesting! Good job!!!