I wasn't aware the full title of this book was so long until I started writing this review...
The diary itself is written by a player called Jason, who is the leader of a party called the Argonauts. While they are fairly powerful, they are not strong enough to be leading the attempt to clear all 100 floors of Sword Art Online. The Argonauts however do fulfill an important role. Because the frontline parties try to clear each floor as quickly as possible, they are not able to explore each floor completely. Parties like the Argonauts are active a few floors behind the frontline parties and help explore every corner of each floor, because they might find hidden equipment, items or hints that may help the frontline parties. It is during one of these exploration missions Jason, and his comrades Orpheus, Hercules, Asklepios, Atalanta and Caenis find themselves transported in an underground labyrinth. While they explore the place, they are suddenly attacked by a menacing Blind Minotaur. While the party display fantastic teamwork, they soon realize that the Blind Minotaur is faaaaar more powerful than they are, and they are hardly able to chip off even a little of his hit points. As a hopeless battle, all they can do is flee, and then soon learn that they are safe if they manage to stay inside one of the few rooms in the labyrinth and keep the door closed, as the Minotaur is not able to get inside "uninvited." The Argonauts try to find a way out, but run into a different party who just made it here: the Legendary Heroes consist of Arthur, Evelyn, Rocky, Azrael, Mark and Omega, but unlike the Argonauts, it's clear this party isn't really a team, and their whole strategy revolves around Arthur alone. The two parties don't really trust each other, and decide to try to find a way out on their own, while avoiding the Blind Minotaur, who seems to have a set pattern of appearing from an underground lair once in a while and then return again. However, the Argonauts are later contacted by the Legendary Heroes with shocking news: it appears Arthur had gone out into the labyrinth on his own during the night, and was killed by the Blind Minotaur (something his party members can see "on screen" happen, as you can always check a party member's hit points and status). There are however suspicious points to Arthur's death, as why did he sneak out in the night and why would he try to take on the Minotaur alone? The two parties agree to find a way out together, and they suspect this dungeon isn't about beating the Blind Minotaur, but finding some kind of secret that solves the dungeon, but while they are trying, more and more people are killed, and they soon start to realize some of these deaths weren't commited by the Minotaur, but one of their own. But why?
As long as it's a mystery, I'm interested, so I occasionally consume mystery media based on IPs I don't even know or have ever seen/read. In the past I have for example discussed the Fate/Grand Order mystery novels by Van Madoy, even though I don't play the game and have still not ever played any of the Fate games or seen any of the anime series. I also originally had no interest in watching Oppenheimer, until someone told me you could totally watch it as a mystery film, and... he was right. So when I learned Konno Tenryuu had written a mystery novel for Sword Art Online, the franchise based on the light novels by Kawahara Reki. I was immediately interested, even though I have not seen or read the series. Yes, I know it's really popular, but I'm from the generation where "an isekai series of a boy having to clear floors to beat a RPG-like world" equals Mashin Eiyuuden Wataru. Anyway, I have read a few of Konno's books too, many of them dealing with fantasy elements, and I like his work, so I knew I wanted to read Sword Art Online Alternative: Mystery Labyrinth - Meikyuukan no Satsujin (2023), or as the cover also says: Murder in the Labyrinth Pavillion.
I can at least say that even with zero prior knowledge of the Sword Art Online franchise, this was pretty accessible. It is, as far as I know, not directly connected to any events from the series, and is mostly a standalone story, though it is set after the original Sword Art Online was beaten, so the first series/book. The mystery plays at two levels, the series of murders that occur in the Labyrinth Pavillion dungeon and the parties' attempts at finding an exit out of the maze, and a meta-mystery where Spica and the narrator try to figure out how this diary could've made it out of Sword Art Online, a world that has been destroyed, and into Alfheim Online.
The murders in the Labyrinth Pavillion are of course the meat of the mystery and I found it highly entertaining. Konno does a great job at actually incorporating the game-element of Sword Art Online to create highly original mysteries, that are truly only possible in such a setting. For example, we have characters witnessing a murder and learning the exact time of a death not because they seeing the death with their own eyes, but because they can see their party member's HP bar dropping in the party member status screen: where else could you ever see such a way to establish a time of death and alibis for the suspects!? Other cool things are how there are no corpses in the world of Sword Art Online, with the player's body simply disappearing from the game world if they are killed, though they do leave whatever they have equiped at the time, again opening up possibilities for shenanigans as the murderer doesn't have to deal with hiding bodies. As a closed circle setting, the Labyrinth Pavillion is also interesting, as the rules of the game make it a place you can't easily get in or out to, making it a very convincing confined location. Add in the rules of how many people can stay in a room and how you have to invite people inside, and the characteristics of the Blind Minotaur (the players slowly learn its attack and reaction patterns through their various encounters, which again tells them how the Minotaur could've been used to faciliate the murders) and you end up with a novel that is just fun to read because you're constantly confronted with new ways to present a mystery due to the unique setting. Clever things are done here, and the fact the Blind Minotaur (a force of nature/a game enemy) is used as a "murder weapon" is of course also really interesting. The motive for the murders is also rather unique to this series: while I don't think it's completely convincing, I do think Konno did a great job at really making this a motive that works in Sword Art Online. The one thing that didn't really work for me was solution to beating the Labyrinth Pavillion: that part of the mystery is just too farfetched, and nobody would be ever able to solve such a puzzle...
Meanwhile, the meta-mystery at the Alfheim Online level is probably easier to guess, as Spica seems to know what the trick is from a very early stage and gives out hints. The answer might seem a bit predictable once you reach about the middle of the book, but overall, I think it provides a nice side-story to Sword Art Online, being completely seperate of the main story, but still firmly set within that world. It probably helps the book was based on an idea by series creator Kawahara, and also supervised by him!
So even without ever having seen any Sword Art Online, I can safely say Sword Art Online Alternative: Mystery Labyrinth - Meikyuukan no Satsujin is a fun mystery novel, that makes great use of its VR MMORPG setting to present a mystery with elements you won't find in other mystery novels. It is a book that fully utilizes the fact it is based on an existing IP, building on the story and rules of the franchise, yet at the same time is not at all dependent on the main story. It's really impressive what Konno managed to do here in that sense.