The premise
The light the kingdom was built on has come loose
The aether was never safe. It was kept, and the Keep is what kept it. When it broke, it started coming up through the floors, and the ground stopped being somewhere you could go.
Tam was on the stairs when it happened. Not chosen, not summoned, just carrying something upstairs at the wrong moment. He kept going because down had stopped existing.
Sera is at the summit. She is the reason the tower is still standing, and she is not going to say so.
There is no villain in any of this. Nothing hunts you, nothing fights you, and nothing dies but your own run. The rising light is a material, not a monster, and the whole horror of it is that it does not care.