In English, proper nouns are capitalized. Common nouns aren’t.
A noun by definition is a person, place, or thing. It could be a physical item. It could be metaphysical. Light is a noun. Clark Kent is a noun.
Nouns like light, love, box, tree, child and yard are fine as is. There is nothing that makes them special and unique.
A child isn’t capitalized. The child isn’t either. Nor is children. They are common nouns. Based on context, you can figure out who or what. We don’t have a specific child being spoken about.
Cyro Hartliebe is a child. Even if Cyro Hartliebe is placed in the middle of a sentence, they are still capitalized. Because now we’re talking specific child.
Proper nouns mean specific. A particular place, person, or thing.
The Empire State Building. Cyro Hartliebe. The Magical Box of Untold Treasures (which you can use in your story or D&D campaign).
Fantasy has a completely made up list of new things, places, and people.
Still it follows the same rules. Specific nouns get capitalized.
In Dragon Rider, I write about The Room. It is a specific place. It has a poor name for a reason. It’s meant to be easily lost in conversation. Same with Other World and the Planes.
They are proper nouns. They deserve the special attention.
‘Natalie walks into the room’ means she’s switching rooms (where ever the rest of the scene suggest). ‘Natalie walks into The Room’ means she’s going to hang out with the living books.
Since the rules are less important than consistency, the main goal is to keep up with your decision.
Fantasy leaves a lot of leeway. Use it. Decide before you hit publish if this needs to be considered a proper noun or not.
Specific and unique are required. But when I name plants and animals on Earth, I capitalize the genus. The giant sequoia is Sequoiadendron giganteum. That’s just another weird rule among the English’s weird rules.
Weigh your nouns. Decide what needs to be called proper and stick with it.
You can’t be wrong when it comes to made up elements. You are the author.