Due to the fact that the Ancient God of Light was suppressed there, the entire space was filled with light that came from the ubiquitous elements of light, so much so that every single corner was well lit. In fact, the people who came here were all enveloped in light too, so none of them had shadows.
However, with Lin Li's progress in drawing the mageweath, a large number of elements of light gathered in the temple. There wasn't a sun in this space, and since the elements of light had been attracted to the temple, the areas where elements of light were scarce naturally became dimmer too.