((Okay, so in basic, any light that COULD BE produced by nature counts as natural. This includes fires, moonlight, sunlight, lava, and mirrors that reflect natural light, if you wanna get creative. This does not include lightbulbs, lamps, man-made electricity, heat coils, etc. If Zechariah is paying attention he can also make small use of the light provided by Lightning also, but this is quite rare.
The power of his light constructs depends on the amount of light available. Full sunlight and moonlight are the best, followed by lightning, fire, and magma. Mirrors generally reflect full light.
If no natural light sources are available, he can create small, short lived light constructs from his soul energy, but this takes extreme concentration and lasts only a second or two. Basically, a last resort. It makes him very tired to do this without natural light, so it would probably be better to use shadows in this situation. If he doesn't have his staff in his hand he can't do this at all.
His basic moves include creating crystal light shards to send flying towards the enemy, shard fragmentation bombs of light, light shield, blades of light guided from his staff, and light crystal plates he can hop on. He can get creative with the shards, but this is his basic skillset.
If you have any other questions, please do ask. Oh, and his power was given by the same entities who controlled The Medium, when all the Guardians channeled their power into him along with a certain other person, giving him this power permanently. And then destroying Artemis' power and all that.))