Four hundred years ago, twelve great warriors drove the greylings back into the Pit and then sealed it with a wall as well as an order of soldiers to watch for their return. Unfortunately, nothing happened in the meantime and the people guarding the Pit grew lax in their duties. No points for guessing that the greylings have returned and now pose an existential threat to the survival of humanity. Can our heroes repulse them? Perhaps but they have to deal with internal struggles and intirgue that threaten to rip them apart before they can.
This book has some similarities in premise to A Song of Ice and Fire but it's not like walls being breached by terrifying barbarians is anything new in fantasy. Besides, this is mostly a much lighter and softer story that has more in common with Tolkien than Martin. The protagonists are good, decent, and hardworking folk up against a race that is mysterious as well as alien in its motivations. There's no hint to what the greylings want other than the total destruction of the surface dwelling peoples of the world.
The primary protagonist is Merad Reed, who is a village boy who went off to join the order guarding the Pit and has had roughly twenty years of walking in circles to show for it. This all changes dramatically one night and he soon finds himself accompanying one of the few remaining Knights of the Twelve in an attempt to warn the closest of the baronies that everything is about to go to Hell.
My favorite of the characters, though, is Jelaïa del Arielum. She is the heiress of Arelium household and someone who has a lot of typical heroine qualities: against arranged marriage, bookish, feminine but slightly rebellious, but quickly charmed me as a reader. Watching her deal with the pile of miseries that results from the invasion is an interesting one. I was glad when the book showed her not being driven to run away to avoid matrimony or other tropes.
My biggest issue with the book is the fact that the greylings receive no characterization and we really never do find out why they want to take over the surface. They can't live in the sunlight so it's like invading lands that are radioactive for half the time you live in them. We also have their human collaborators that I'm not really sure the motive of makes sense. I hope future books in the series will go into their motivations more.
In conclusion, this is a decent fantasy novel and has a lot going for it. It uses a lot of tropes but mixes them up in ways that I found engaging. The characters are entertaining and the action is well-described.