It's likely about a cartel named 'the frogs' (literal translation). Since it's based on a true story, it might involve real - life events related to a criminal organization, such as their operations, the people involved, and the impact on society.
There could be a young and curious character, maybe a novice in some sort of magical or historical study. This character is drawn to the mystery of the nagas and sets out on a quest. There might also be a group of protectors, whose job is to keep the secrets of the nagas safe from those who would misuse them.
One of the main themes could be mystery. Nagas often carry an air of mystery in many mythologies, so perhaps the novel explores that mystery. Another might be adventure as the characters might be on a journey to discover the secrets of the nagas.
A 'historia visual novel' could be about various things. It might be a story set in a historical context, with characters and plots that unfold in a way typical of visual novels. It could focus on a particular era, like the Renaissance or the Victorian age, and tell a fictional story with elements of that time's culture, society, and events.
Well, perhaps one key element could be their habitat. If nagas are associated with water, their underwater lairs or the hidden places they dwell might be part of the secret.
Yes, there are quite a few. You can start by looking on fanfiction.net or Archive of Our Own. Some fanfictions might explore their relationship in an AU (alternate universe) setting, like a modern-day scenario where they meet and get married, while others could be more in line with the original story's themes and settings but with a different plot twist leading to their marriage.
There could be some, but it really depends on the market and the fan base. Since visual novels are a diverse genre, those with a historical focus might not always get the same level of mainstream attention. But in the communities dedicated to visual novels and historical fiction, there may be some well - known 'historia visual novel' titles that are highly regarded for their storytelling, art, and historical accuracy.