Nocturnal Illusion is a Japanese bishōjo

Bishojo game

A , or , is "a type of Japanese video game centered on interactions with attractive anime-style girls". These games are a sub-genre of dating sims targeted towards a male audience....

 eroge

Eroge

An or Ero-ga is a Japanese video or computer game that features erotic content, usually in the form of anime-style artwork. Eroge originated from galge, but unlike galge, they feature erotic/pornographic content.-History:...

, developed by Excellents Japan. It was translated into English and released in the United States in 1997. JAST USA

JAST USA

JAST USA is a licensor and distributor of English-language bishōjo games, some of which are as dating sims or eroge. It was founded in 1997 with Sakura Soft's Season of the Sakura and JAST's Three Sisters' Story as its first eroge titles...

 later acquired the rights to re-release the game after the original localization company (RCY Brothers) had gone under.

In 1998, a 2-CD revised version of the game with better graphics, sounds and this time with characters voices was released in Japan, by Apricot. It was never officially translated into English, but ViLE can be used to combine the English scripts with the resources from the remake. The game served as inspiration for an amateur adventure game

Amateur adventure game

An amateur adventure game is a computer game belonging to the adventure genre that has no commercial or similar official backing. The amateur adventure game scene emerged in the mid to late 1990s, when releases of new commercial adventure games became more rare and easy distribution of games and...

 5 Days a Stranger.

Gameplay

In this, the main character decides to get away from it all during his spring break and stay in the mountains. While staying in the mountains a massive storm starts leading to the main character fainting. A mysterious woman saves him and uses sexual pleasure to keep him warm. It turns out that he is now in a mansion that is outside of regular time and space, with a gate that only opens when it chooses (while not explained who does it, food among other things are left at the gate by some unknown force). The woman who saved the main character is the mistress in charge of the place. The main character decides to explore and meets many women, which leads to sexual experiences with all. By the end of the game, the gate opens and the main character can chose either to stay or take one woman out of the mansion.

People living in the mansion

Shin'ichi

The hero in the game. The name can be changed. He arrives at the mansion after encountering a typhoon. He must escape the place to win the game. At completion of the game, he can choose himself one girl as his partner to leave the mansion.

Mistress

The mistress in charge of the mansion, she has an almost psychic ability where she can tell when someone will arrive at the mansion in advance and always seems to know what's going on.

Miwako

The mansion's maid and is somewhat shy. When she was 14 she was sent away from her family due to they were poor and she was sent to be a servant to a rich family (The main character remarks that this makes her quite ancient, though the mysterious mansion keeps her young).

Maya

A journalist who has had previous bad experiences with men and despises them (due to overhearing people saying things like "she's just a woman" and that she should become a house wife). Though slowly the main character convinces her otherwise.

Misao

A 18 year old girl (in the English version) who grew up in the mansion and wasn't raised by anyone so acts very childlike and is extremely innocent in what she knows (even to the point she doesn't know about sex).

Sari

A mysterious woman who waits for a lover who has long since left the mansion. She is truthfully a supernatural creature who lives on blood and transforms into a flower during the day.

Yukina

A very shy woman who spends most of her time in her room. It is revealed that when she was found by the mistress outside the mansion, she had attempted suicide but was regretting it deeply screaming "I don't want to die."

Yura

Spends most of her time in a temple-like building, guarded by a demon.

Arisa

A young girl who is older than what she appears.

Red Riding hood

Assaulted by a wolf; the main character saves her.

Mermaid

Lives inside a well but is saved by the main character who helps her return to the ocean. This character is based on the tale of the little mermaid, but somewhat more sinister in some parts. While she made a deal with a sea witch to go onto land to see the prince she loved, the prince loved someone else and as per the witch's deal, it was either the prince falls in love with her or she would turn into sea foam.

While unaware to the main character feels kicking inside her, she was pregnant with the prince's child and so the witch made another deal. The fate of turning into sea foam would be removed if she would kill the prince with a pair of scissors. She does this and ends up in the well.

Kusayama

A man who after drinking too much ended up in the mansion. He invites the main character to drink in his room leading to them both collapsing from the amount of alcohol drunk. Some of the other residents of the mansion dislike him due to his pervertedness. He attempts to rape Maya, then Yukina, but is stopped by the main character.