First, study the existing world of Makai Ouji. Know the rules about devils, their powers, and how realists interact with them. For example, if devils have certain hierarchies and magic systems, your OC should fit into that framework. If your OC is a devil, they should have powers that are consistent with what we know about devil powers in the series.
One idea could be an OC who is a powerful mage from a forgotten lineage. They could be brought into the world of Makai Ouji to help balance the power between the devils and the realists. Maybe they have a special ability that can temporarily nullify devil powers, which makes them both a threat and a valuable asset.
Well, in a 'naruto ruler of makai fanfiction', it could center around Naruto being transported to the Makai and having to rise to power. Maybe he has to face off against powerful demons there and use his unique abilities like the Rasengan and his shadow clones in new and unexpected ways. He might also have to form alliances with some of the more friendly creatures in Makai to solidify his rule.
Well, it could be about Naruto in a makai (demon world) setting, perhaps becoming a king in that realm. It might involve Naruto using his ninja skills in new and strange ways against demons and other makai creatures.
The 'devils' in 'devils guard the real story' are very much a mystery without further context. They could be the product of a creative mind's imagination, designed to add an element of danger and mystery to the story. They might be inspired by traditional devil - like figures from mythology, such as Lucifer in Christian mythology, with all the connotations of evil and temptation. Or perhaps they are a new and unique creation within the story, having their own set of characteristics and motives that are yet to be discovered. Maybe they are not even living entities but rather some kind of inanimate objects or forces that are personified as 'devils' for the sake of the narrative.
I'm not entirely sure as I haven't read it specifically. But generally, a light novel often features interesting characters and unique storylines. It might be about a 'tensai ouji' which could be translated as 'genius prince'. It could involve his adventures, his struggles in a fictional world, or his relationships with other characters.
Since I don't have a deep knowledge of this particular novel, it's hard to say precisely. However, based on the title, it may tell the story of a prince. The word 'eiyuutan' might imply a heroic or epic journey. So, it could be that the so - called 'akuyaku ouji' is on an unexpected heroic journey, facing his inner demons and external enemies, and growing as a character throughout the narrative.
One horror story is about a group of teens who used the Ouji board. Strange noises started in the house right after. They heard scratching on the walls and doors slamming on their own. They were so scared that they threw the Ouji board away, but the hauntings continued for a while until they got a priest to bless the house.
No, I don't think so. Most of these stories are just the result of people's overactive imaginations. When you play with an Ouji board, your mind is already in a spooky state, so you might misinterpret normal things as something paranormal.
Obviously Naruto would be the main character. But there could also be other characters from the Naruto universe who might be transported with him or have some connection to Makai. Maybe Kakashi could show up to offer his guidance.