Book about a monster that leaves the skin.

As grim as it sounds, I am desperately looking for this book. In terms of it's appearance, the version i remember was a solid dark blue with rose gold designs and lettering. To ofc different editions can look different.

i didn't actually finish the book. What I do remember was that it was definitely fiction, an a YA book. Anyway, I think it had a few main characters who were children or teenagers. And they had to act as guards for their small rural town (like dark ages England) that was surrounded by mores, there was also maybe a wall. The key thing I remember though is that the guards were there to protect the town from a monster (that wasn't yet described in the past I read) that would kill the guards and leave their skin like a fully intact suit. And I think it attacked at night and lived in the more.

Please help new find this, abs I'm grateful to any and all suggestions. Like even if you think it only matches one of the things I've mentioned LMK thx