Those events aren't zombie attacks -- they're mysterious deaths breaking out in various locations throughout Great Britain during World War One. Have the Germans landed some sort of hidden force on the British Isles? Is someone using a mysterious 'Z-Ray' to smother people or send them running off cliffs to their deaths? Or is there something wrong with the animal kingdom?
Machen was writing furiously at this time in his life, forced into newspaper work in order to pay the bills. The Terror isn't the imaginative and literary triumph that the aforementioned stories were, but it's still an enjoyable and often weird book. It's also an important permutation in horror's long love affair with the pseudo-documentarian style. Where 'letters' and 'journal entries' once told us that what we were reading was 'real,' now the journalistic voice does.
It's also a mutation of something going back to at least Daniel DeFoe's A Journal of the Plague Year. In that early 18th-century work, DeFoe told a fictional 'you-are-there' story about a real event. Machen embeds The Terror in the real, early events of the Great War and then, like DeFoe, tells everything as a piece of actual reportage. It's a major stylistic leap.
There are many fine moments of horror and pity throughout The Terror, along with some marvelously weird images. Machen captures the way fear can travel through gossip when the official channels are trying to hide the problem. A late-novel tableaux of horror at an isolated farm is especially well-managed through the description of the aftermath and through a dead man's journal describing the mysterious and terrible events that led to that aftermath.
There are a couple of flaws to note. One isn't so much a flaw as a relative lack of closure. Things just sort of stop. This first flaw is exacerbated by the second, which is the narrator's jaw-dropping, climactic theory about why what happened, happened. It's an explanation totally in keeping with Arthur Machen's beliefs about society. But it's a moment of political and social commentary that will leave a sour aftertaste with anyone who doesn't long to live in a medieval fiefdom. I kid you not. Recommended.