The Decagon House Murders is a manga adaptation of a novel by Yukito Ayatsuji, probably better known in the manga community as the original author of the horror series Another. The art is done by the unbelievably talented Hiro Kiyohara, the artist behind the manga adaptations of Another, Kizu, Calling You, and Tantei no Tantei.
The original Decagon House Murders is, in my opinion, one of the best-written novels of all time. There are no wasted lines, the characters feel realistic, the murder tricks are simple yet clever, the atmosphere of slow terror settling over the title Decagon House is perfectly captured, and the ending ties it all together into a neat package that lends even more characterization to what is already one of the best characterized murderers in all of fiction. Though the manga deviates a bit from the source material and in some areas suffers for it, it's an overall well done adaptation with stunning art and areas where it even improves upon the already incredible novel.
The novel version kicked off a genre called "honkaku" mysteries that are written as both a mystery and a sort of game to see if the reader can solve them. As a character in the novel puts it, it's a sort of "intellectual puzzle. An exciting game of reasoning in the form of a novel. A game between the reader and the great detective, or the reader and the author. Nothing more or less than that.” It's very much modeled after Golden Age mysteries such as Agatha Christie's And Then There Were None, and makes direct references to them throughout the book. In short, it's a sort of love letter to mysteries that invite the reader to be not only entertained but to also exercise their wits, and it's an excellent one.
Enough about the novel. How does the manga adaptation stand as its own work? Quite well, in fact.

The Decagon House Murders is a classic "chateau in a snowstorm" setup, as a character aptly points out. A group of college students, part of a university mystery club, travel to an island, which had previously been owned by an eccentric architect. He'd built two buildings there, the so-called "Blue Mansion" and the "Decagon House". A fire had burnt down the Blue Mansion a little while earlier, with the bodies of the architect, his wife, and the two housekeepers found- murdered before the fire had broken out- in the burnt-out ruins. The gardener had gone missing, and the question of what had happened remained. When murders start to occur among the students, the mystery takes on another layer: is one of the previous occupants still alive, or is there a murderer in their midst?

The story is told in parallel, with the group of students on the island as well as another group on the mainland, investigating strange letters sent to members of the university mystery club that seem connected to the original murders. It's an extremely clever way to give the readers more clues without giving away too much of the story, and the contrast between those on the island growing more and more scared and those off the island treating it as a game keeps the story from growing stale.

The manga adaptation did make some weird decisions that didn't particularly work. For one thing, it's set in the modern day (the original was written in the 1980s). For the closed environment to work, it had to have a whole thing about cell phones not getting reception, and that was just kind of odd. The time also wasn't really important at any other point, so I'm left wondering why the change was made. Perhaps to be more accessible to modern readers, but frankly anyone actively reading the series who was going to buy it or vote for it on reader surveys probably was a mystery fan and familiar enough with 1900s-set stories to not have it bother them. There's also a major plot point in the novel that makes the story more confusing after the characters find out about it, but in the manga it's conveniently revealed easily for no apparent reason, and it only serves to ramp up the tension a little bit as well as make a massive change to Ellery's characterization compared to the novel. It's still well done, but I personally thought it was an unnecessary change that didn't add anything to the story.
There's also a set of changes that change the characterization of the murderer, which worked out decently. At the very start of the novel, there's a scene where the murderer puts a letter detailing the murders in a bottle and throws it into the ocean while contemplating what they're about to do. They acknowledge their insanity, their motivation for the murders, and that "Man cannot become God" even if they're "playing God" and sentencing others to death. It's an extremely beautiful scene that sets the tone for the novel and provides a ton of characterization for the murderer, and in the manga adaptation it's been split into several parts and kind of scattered throughout the series. It reduces the impact of the scene and the start of the series by quite a bit, but isn't a story ruining change. The motivation for the murder has also been changed slightly in a way that I think actually added to the character of the murderer quite a bit, and allowed the last few chapters to focus on them and how the murders were carried out in a way that added some depth the novel didn't have. Overall, it worked out pretty well, and the murderer remains well-written.

As a mystery, The Decagon House Murders does hold up. The air of fear and tension present in classics such as And Then There Were None is great, those off the island have a fun buddy-detective duo kind of vibe that's super fun, and the tricks used to carry out the murder are realistic and clever. The characters react to the murders in ways that feel pretty human, and the characterization for a few of them, particularly the murderer, is top notch. The artist managed to portray some events I had thought would give away the tricks if shown visually without revealing too much, which led to some extremely impactful panels. The novel originally leans on how ridiculously good the murderer reveal and characterization are, and the manga manages to keep that while still delivering a great experience that gets better every volume. In short, a great adaptation, even if the changes take out some of the impact that the novel had.

Oh, and as a spoiler-free example of how insanely good the art is:

Ultimately, The Decagon House Murders manga doesn't quite stand up to the novel (which I would recommend reading as well), but as a standalone experience it's still an incredibly good mystery manga. The characters are relatively realistic and fleshed out, the murder tricks are clever, the atmosphere is well created and the art is some of the best in the industry. The adaptation still manages to be one of the best comics I've ever read, and I'd recommend it to any mystery fan.