Catastrophe at Sixteen is a prequel to the main story and shows Guren’s life in his freshman year of high school right before the catastrophe that ended the world (with flashbacks to his childhood). You watch him as he makes friends (who we know as the Guren squad), experiences love, heartbreak, grief, and the start of the loss of his humanity. These books give a lot of background to his friends, especially Mahiru and Shinya.
Kagami started his career in writing novels, and it really shows here. Although I love the Owari no Seraph manga, the way he writes his novels makes everything more emotional and personal. Guren and his friends feel way more relatable. There’s a good amount of scenes where they feel like real teenagers. Perhaps I’m biased, so take this with a grain of salt, but the Guren squad is much more enjoyable than the Shinoa squad - for one, they actually started as friends and weren’t just forced into a military squad; two, the Shinoa squad hasn’t had enough screen time (page time? IDK) to fully develop. I think anyone would enjoy watching the way Guren & Co. interact. If you’re a fan of Mika, Yuu, Shinoa, Ferid, or Saito, don’t worry, they appear in some chapters too.
If you read the Owari no Seraph manga and have questions, you should really read the novels. They don’t answer everything obviously, but they answer a lot and offer plenty of evidence for new theories. Even if you don’t enjoy Guren’s character, there’s a good chance you might still like it - these books made Shinya one of my favorite characters in the whole series. General warning that there are some scenes that talk of topics that can be upsetting to people, such as mentioned r//pe, child ab//se, and Mahiru Hiiragi being Mahiru Hiiragi (lmao). That said, I didn’t find it to be too graphic, but just in case you needed a warning.
