I found myself in a similar position years ago when I also reviewed the part two of a twin album and explained in it my despair at not being able to cleanly approach each of them without the context, part two because follows something, part one because I could only do so retroactively. That is especially frustrating when the twin albums are tied only narratively and thematically and don't otherwise offer any contrast between the two, like, for example, having one side be harsher and the other mellower. Just like then, Zwart Vierkant and Zwart Vierkant: Slotstuk are both avant-garde black metal albums, neither being fundamentally different in sound from the other. And that's the kind of thing that only works when the concept part of the concept album is so tied to its themes and narratives.
And the narrative in Zwart Vierkant: Slotstuk and its predecessors is one that is so beautifully meta, adapting the story from a novel that founding member Ruben Wijlacke wrote titled "De protodood in zwarte haren" (Dutch for "The protodeath in black hair"), in which the main character named "Pedro", a 20th-century Modernist painter, becomes fascinated by Kazimir Malevich’s Black Square, which is, as the title suggests, an abstract painting of a black square. Pedro convinces a group of artists to take that radical idea even further. So we have a music album based on a book based on a painting. Of course that's all a bit hard to gather merely from the music itself (more from the cover arts), especially since the lyrics are in Dutch so the language barrier makes the concept indecipherable for most people, but you can read all about it on the band's webpage.
This being a Dutch black metal album, a scene renowned for its incestuous sharing of members, one can expect at least one member from another recognizable band. Grey Aura spent their first decade as an offshoot/continuation of Folkstorm (which didn't hit the mark), but lineup shuffles since have led to the introduction of drummer Seth van de Loo of Severe Torture, and more relevantly, of bassist S. of Laster, and it's the latter whose bouncy grooves do leave a mark that grounds the sound in something more familiar, though they're not as pronounced as they are in Laster's music.
The Dødheimsgard meets Hail Spirit Noir skeleton of very rhythmically flexible, somewhat psychedelic and disorienting dissonance, ever shifting structures, moments of expanded instrumental palette containing mellower acoustic passages (that do actually work as more than just cleansers) or cellos, trombones, and tubas performed by guests, all make the duology a brain scratcher, though it is this second half that feels dramatic in a way that's stripped of all the joviality associated with the term and that's sometimes inherent in avant-garde black metal, usually in moments that use theatrical cleans or circus-like instrumental structures, none of which appear here. Zwart Vierkant: Slotstuk is an album I would describe as "maddening" or "manic", but you can tell Pedro isn't having any fun losing his mind.
The duology and its separate parts are best enjoyed in tandem with their concept, but the listening experience is detached enough from it that one can easily listen to each of its parts without taking anything more than the music itself in and enjoy them as great pieces of somber disorienting black metal, the latter more somber than the former.
