To get this out of the way: This is not a mystery visual novel, and even calling it mystery "themed" is questionable. This is a drama that unfolds with some twists, both unexpected and hinted at--as you would, you know, expect from most stories. (And while I would love to continue on my soapbox asking why the majority of visual novels released in the west are mysteries, with little room for much else (aside from hentai), I will desist.)
Genre dickery done: The House in Fata Morgana is a fairly well known visual novel. It's a straightforward storyline, the main diversions in your path being choices leading to either dead ends or one of seven false endings, the eighth being the true one. There are a couple of instances where you have more than one choice, but these are for fun and the story moves on regardless of what you pick.
You start immediately in the titular... mansion, where you meet The welcoming Maid. From there, she guides you to various doors, which each contain their own story. They're all tragic love stories, set in different years with different characters, yet all connected to the house. This could be considered the prologue, so while I advise people to read them seriously because it is important later, they are a touch melodramatic. After these, the game really kicks off. I'd say these stories collectively are about half the game; the rest covers who you are, who The Maid is, and the true nature of the mansion.
This is where the game comes to life. It could be slimmed down a touch, but the stories told in this section are effective and shouldn't be spoiled. Like the earlier parts, it's a love story at the core and the game is ready to assault you with sadness for a good long time. It's very effective (unless you're some weirdo who has never felt unlovable), and while always feeling like a story you are separate from, at times it cuts deep. It's never bad as like, watching your favorite absolutely miserable movie, but it gets close. Some of the particulars might be tough for people with certain pasts and anyone emotionally tender should tread lightly (I think all topics are handled well and said people would enjoy it in the end, but there are some gut punches for sure). However low it gets the game is on your side, and thankfully eases up into a thoroughly satisfactory conclusion.
I recommend stopping there!
When the main game is done, new stories will open up, one after another. There are about a million of them. None felt particularly worthwhile to me. I ended up force skipping through the longer two (the one set in the oldest and most recent eras, to not give too much away). The game already has a habit of leaving nothing to the imagination, and these only spell out more and more. Are they detrimental? I dunno, I just find them unnecessary to an otherwise good game.
Aside from that, my biggest criticism is how the writing doesn't have much of a style when the game in most ways stands out from typical vns. It often reads like a stiffly translated story (as opposed to say, Higurashi where the overwritten style adds to the atmosphere). But I think it's just written like that? I don't need anything crazy, but some style could've elevated this into amazing status. Along with that, as you jump between timelines there are no language differences. People from 1800 and 1600 speak exactly the same, and usually as casually as we would today. I don't want inappropriate thous and thees, but a little more formalizing of speech where appropriate would give the different eras a flavor of their own.
Visually, this is a unique vn, and it's a huge highlight. The characters are expressed through a delicate gothic style with realistically rendered color and shadow. Everyone has heavy eyelids and well formed lips, dressed in period clothing (more or less) and set against backgrounds which are layered, filtered, and noisified to set it apart from reality. (To bring Higurashi back up, it's not simply blurry photographs.) The sprite style is carried well into the CGs. Characters who aren't as important don't have sprites; it's not like vns where they toss in a generic human image for side characters, and I appreciate them keeping the style intact that way.
These sprites are used in most of the side stories, but not two: in one there are new sprites in the same style and they're quite nice. If it's not the same artist it's very close, perhaps with a few years of refinement. However, the other is a bizarro world where they're replaced with the most generic anime sprites and backgrounds. It's a huge downgrade. There's also voice acting! Instantly muted! Stripping uniqueness out of a good game and repeating the main story but watered down? I can't tell you to avoid this section enough (too bad even its title is a spoiler!). If you see anime, run. (Or force skip for the trophy...) (Words of a moron.) (Ok to unlock the gallery you have to play through everything so it was worth it to me.)
I don't often bring up music in reviews because I don't know how to talk about it, but I recommend listening. A lot of the tracks are music with nonsensical female vocalizing (no it's not like Nier) in a variety of styles, from jazz to... uh. Sad music. In case you didn't get that the game is kinda sad. The maid and mansion themes will get ingrained into your brain, and they are sometimes used in surprising ways, or as motifs later. Thankfully I didn't play on Switch because I likely would've muted it and the soundtrack should be heard. (A random reason to like it on PS4: during a scene the light on my controller suddenly switched to red! and I knew a split second before text appeared on the screen that shit was about to go down.)
I would recommend this game to most people who like vns, and I think it's a good "starter" vn for people getting deeper into the genre who don't want to commit to a full sound novel just yet. It's not ridiculously long, the story maintains itself well, with very likable (and hateable, in a good way) characters, and there are relatively few stupid anime moments (there are some), especially if you stick to only the main game. If you must play the side material, give yourself some time as it's hefty and I think even fully invested people might get tired of it.