So What Is the Deal With the Wellspring/Where does the light come from?? - Descent DLC

I always thought it was lyrium. Either lining the high walls or a super bright deposit near the top of that cavern. Or perhaps super potent lyrium vapors.

I thought it was lit by lyrium too. But the Titan is so big it seems to have its own weather system too

It is the true lyrium causing the brightness, the lifeblood of the Titan. In the previous chamber it was literally flowing down the walls and into vast lakes of the stuff. It is why I couldn't understand why the mages in our party weren't bleeding from the eyeballs, since lyrium is meant to be dangerous to be around but may be that is the mineral that the dwarves mine, which is impure and crystalline.

The codex that goes with it talks about a dwarf king tunnelling through to the sky, which may be accounted for by the cloud and the brightness making him think he had gone all the way through to the outside world.

Here's another thought though. In more than one place in the Chant of Light, the Maker is referred to as the "Wellspring" and the Maker is associated with light. So does the Titan have some connection with the Maker?   Even the Chantry believe that the mineral lyrium is the essence of all Creation, left over from the creation of the world, and some people call the sound it makes the "voice of the Maker".

It comes from the sky. No, really.
There's a sky on the bottom of Thedas, too. That's why Dwarves have that weird superstition about being able to fall into it.