City of Light

In the Plane of Shadow lies a city of light. How? It's a mystery the players might crack. The light from the city walls repels demons, undead, and other shadow creatures. They have a farm that grows with magical sunlight emitted by glowing orbs. How is this possible? The city folk speak in reverent tones about a God of Light that grants them these miracles. But something seems strange. These people seem brainwashed. This city is definitely not what it seems.