WMG / Knights of the Borrowed Dark

Malleus's hammers are made of Knights' iron

The hammers and the Cost are both described as shineless black iron. Furthermore, the fact that Mercy's cage was made from Knights' iron and nothing else, and that a Knight had to build it, could mean that iron from the Cost has some sort of harmful effect on the Tenebrous—maybe that's why they are the only weapons that can kill certain Tenebrous.

Cants are Tenebrous

When Mercy gives Denizen knowledge of the Cants, he senses something else about them, and how they fit together—this is it. The Cants are a type of ancient Tenebrous that slip into the world when called on.

A Knight's Dawning is actually a Breach, and the body the Tenebrous find for themselves is their host Knight, who they form a mutualist symbiotic relationship with. Before his Dawn, Denizen said he felt like he did before the bad-dream angel, and afterward “it felt like something inside of him had broken and could never be repaired.” This is the description of a Breach.

The power of the Tenebrae feels almost alive, as do the Cants themselves, because they are. They're concepts, yes, but they're also Tenebrous that can be accessed utilizing the ones that live inside all Knights. Tenebrous don't have definite forms before the enter the real world—these Tenebrous' forms are Knights and Cants.

This is why Mercy knows all the Cants, and better than the Knights do. Why she talks about the knowledge she gave Denizen like it's a physical gift. Why messing up a Cant can cause a Breach. Even the clockwork growing out of Grey's hand can be explained by this!

Grey is going to die...

and it's going to be by way of a Heroic Sacrifice to protect Denizen, or a Mercy Kill (in which case he'd be the Irony Knight).

The Endless King is the Tenebrae

It was a theory presented in-universe—”Some say he is the Tenebrae, and all the Tenebrous his stray and hungry thoughts, bleeding over into our terrified world.” With the revelation that the King's heart is a Tenebrous called Mercy, this is looking pretty possible.

Knights literally are angels

Seraphim Row, anyone?

Knights used to be Tenebrous

It was suggested at multiple points throughout the book that the Tenebrous and the Order might have worked together at some point in the past—what if that's because the Order of the Borrowed Dark used to consist of Tenebrous?

Something like this.

An extremely long time ago, the Endless King and Forever Court worked together with a faction or Earth-dwelling Tenebrous called the Order of the Borrowed Dark to fight a common threat. Together, they developed a system of Cants that channeled their powers in the real world.

Once this threat was defeated, the Order stayed in the real world as surveillance of some sort. They took on human-like forms, as ancient Tenebrous can, but of course, they weren't perfect— the eyes, the face, etc.. But as centuries passed, they evolved, and mixed in with humans, until they were physically nearly indistinguishable.

And they drew further away from the Tenebrae as time progressed. Fighting other Tenebrous, as their kind is prone to do, but drifting away from the Endless King as well. They eventually forgot their origins, believing themselves to merely be humans born with a connection to the Tenebrae. And while the real world had long been a feeding ground for Tenebrous, the Order began to fight to defend it.

As they became more and more human, they became unable to use their Cants as effectively. The power of the Tenebrae was unnatural, infected their world, and their human sides fought against it.

So it left a mark. A Cost.

If they were to use their power, let the Cost devour their body, leave no place left for their humanity to fight for control—

They'd revert back to their origins.

The Clockwork Three have killed every other Hardwick

The Hardwicks are very closely connected to the Order, and have been around for centuries. But Vivian and Denizen are called “the last two Hardwicks,” and it seems kind of odd that they'd complete a “full set” if the only other Hardwick the Three have killed is Soren, Denizen's father.

What if that's because the Clockwork Three have been hunting down the Hardwick family for a long time, even before the events of eleven years ago, and have killed all other members of the family?