Fucking hell. This is a nightmare.
It's a clichè to describe something as an experience, but Imiri Sakabashira's The Box Man is the real deal. The story is straightforward at first, as our Kappa-Cat joins the titular character character for a ride. There are teases throughout the first forty pages of unexplained menaces -- a small kaiju that springs up out of a river and breathes some type of mist on our heroes; crab pincers that pop out of the box, cutting off the head and testicles of policemen. However, as the Box Man and Kappa-Cat arrive and go farther in a city-wide apartment complex, all hell breaks loose. After making a dealing with a shamanist-like character, we see a montage of rooms where all sorts of nasty monsters are mutilating, molesting and murdering human beings -- only for the reality of the situation to turn on its head at the end of the sequence. After this centerpiece, shootouts and chase scenes segue from one to the other, until we get to a tranquil ending... but not until Sakabashira decides to gobsmack us with an almost casual revelation about the box's contents, with an even more unnerving final page.
Despite the rather restrained portrayal of action, the art is far from minimalist. The two outdoors sequences that bookend the story are rendered perfectly (a subtlely unnerving landscape of industries and old motor vehicles, contrasted to a liberating market that leads into the ocean), but they only serve to heighten the intensity of the apartment complex. Endless wires and corridors with doors peering over the characters, an open river to darkness, sunlight never getting through -- it's a claustrophobic mess. The repetitive sound effects only add to this, elongating the feel of time that the our protagonists stay in this hellhole.
There's absolutely nothing else like it in manga; the best comparison I can make to any kind of comic is that it's a more technically skilled, less juvenile version of Rory Hayes's work. If you want the fucking strange and have an interest in the grotesque, you need to pick this up.