Not quite a prequel or a sequel, but a mishmash of stories that take place before, after, & in between, like a collection of Lost Chapters Nagai decided to add long after the series ended to explain things in more detail & create origin stories.
The story is erratically non linear, panning back & forth between pre-ice age & post apocalypse after just a few pages. The original Devilman manga is mandatory reading to understand what's going on.
Unintentional humor: Dosuroku still looks like Popeye the Sailor Man. Kaimu bears a strong resemblance to both Thor & Baltard from the Galaxy Warriors action figures. Apparently T-Rexs are the only dinosaurs that exist & they traveled in very large packs, except when they didn't.
A totally different artist from the original. The art is a weird mishmash of hyper-realistic heads on extreme idealistic bodies, at least the artist's ideals. The women are extremely bulky & muscular with elephant limbs & gigantic balloon tits. Satan straight-up looks like a blonde 90s Marilyn Manson with wings & oversized hands. Xenon, Akira, & pre-transformation Ice Age Amon are lipstick-bishounen to the 11s, having the prettiest girliest faces, brought down by their huge clown feet. The gore & landscape destruction are where the art really shines.
While the art is objectively fantastic, it was just too distracting. I feel like the artist would much rather be drawing porn than drawing this. Nagai's nudity was at least tasteful, but there are entire panels of nothing but T&A close-ups that try to distract you from everything else. He'll even hoist breasts all the way up to the bottom of the neck where they look stupid if the panel calls for a closeup of a face. The way he draws arms way too long with gigantic hands on normal people is also rather annoying. Lastly, there's the inconsistency of the fashion; In the school chapters, the men are appropriately dressed in 70s clothing while the girls are dressed like out-of-period streetwalkers.
Because of the mixed feelings I had on the art, I found myself wishing that Nagai had just drawn the series himself & then rereleased the original series with all these loose chapters added in in chronological order. I hope to reread both & figure out exactly where to place these 6 long stories.
Characterization was every bit as hit & miss as the art. Dosuroku gets more fleshed out in this series, though I'll never understand how he went from violent bully to gold-hearted kissass. Miki gets a character reduction, being reduced to a bland horny love interest. Akira's personality is more hardass & perpetually annoyed & less noble or idealistic. The two ice aged women who were never given names had marvelous character development.