Leona says
"Remember the first riddle we found and couldn't make sense of? It's in the room on the other side of the main hall."
She leads you through the Library, across the top of the main hall and into the room on the other side. The stone slot and candleholder are in here. You put the bem into the slot and light the candles. You return to the main hall and see that the Mural has been restored to it former glory. The stairway down has opened up again.
You head down and find yourself in a small room. There is a tall door heading north and a strange machine in the northwest corner. It looks like this.
You feel a strong dark presence to the north.
Leona comments
"Do you feel the Immortal to the north? We better go in ready. That machine there is called a solar station. It was used to store solar essence in the Solar Empire. I wonder if it has any left after all this time..."
She walks up to it and sicks her hand out to it. Sunlight essence flows from the machine into her hand. After a minute she stops.
"I'm full now. There is a little left if you want to fill up."