Knights of the Borrowed Dark is a book about the secret war between a species of extradimensional shapeshifters called Tenebrous and the order of Knights who fight them off. Tenebrous enter our world through the shadows, tearing a hole in the fabric of reality, climbing through, and pulling together a body from whatever they can find, whether that's concrete, sand, metal, flesh, light, or anything else.
Some humans are born with a connection to the Tenebrae that manifests when they turn 13, a fire that blooms in their hearts and latches onto their emotions. It's very hard to control and wants to be used, but every time it's used a bit more of their body turns to iron, starting from the palm of their hand and spreading out from there. In order to lessen this, Knights use Cants, words in an alien language that control and harness the fire of the Tenebrae.
The main character is 13-year-old orphan Denizen Hardwick, who is suddenly contacted by a mysterious aunt he never knew he had. It turns out the orphanage was given a fake birthday for him, and when he turns 13 that night, his connection to the Tenebrae Dawns and he is faced with a decision: learn to control his powers enough to not be a danger to himself and others then go back to the orphanage and try to live a normal life, or give up the future he thought he had to lay his life on the line day after day for the sake of people who will never even know he existed.
Meanwhile, the end of the world looms heavy on the horizon as the King of the Tenebrae threatens to Breach if the Knights can't find his stolen heart, and the past Denizen's aunt refuses to talk to him about might repeat itself all over again.