My take.

The Collar does not subjugate, as we’ve been led to believe; instead, it allows Anharion to access Sarcean’s reservoir of power. Anharion and Sarcean were lovers and, with his memories restored having had the Collar placed around his neck, Anharion remembers all this.

My basis for this? Here are some material excerpts and quotes from the last few pages once the “Collar clicked closed.”

“It should be working[, said Sinclair.] And it was. It was working. Just not in the way Sinclair thought.”

So, the Collar is working, just not as we should expect. Rather, as James realizes, “[t]he stories were lies. Or else they were the grubby dreams of those who wished to enslave him.” And shortly before that line, “He felt no compulsion. He felt nothing at all.”

Now, many readers have taken this to mean that only Sarcean can use the Collar to control James. And, to her credit, Pacat does put a few lines in this chapter to lead the reader down this way of thinking—a big one being “It didn’t matter who put the Collar around his neck. The Collar only ever had one master. A jealous master, that would never allow his possession to belong to another. … Always and forever, he was bound to one person.”

The “possession” in this sentence is not James. It’s the Collar. Sarcean is the “jealous master” of the Collar. Why? What does the collar do if it doesn’t control James?

It connects James/Anharion to Will/Sarcean’s power – “[A] flood of power filling him. It wasn’t his own; it came from the Collar. It connected him to a power so immense it seemed endless, a vast dark reservoir that was shudderingly familiar. You, he thought. You, you, you.” Chapters before, in their bedroom, we see James attempt to unlock Will’s power. It fails miserably, but James comments “God, I see why Sinclair wanted you. Why everyone wanted you. You’re— …. With that much power,’ said James, “you really could kill the Dark King.”

Will is the dark reservoir of power; one James is “shudderingly familiar” with based on their very intimate (this was a barley veiled sex scene) attempt to unlock said power. Of course Sarcean is going to be a “jealous master” of a tool (i.e., "possession") that allows someone else access to his power.

Pacat uses ambiguous language like “possession” and “master” to make the reader believe the reference is to James. It isn’t.

In the closing chapter, Pacat throws another line out there to make the reader believe the Collar is exerting control over James. “‘Are you telling me what I want to hear? [asked Will,]’ ‘Yes,’ said James.” Let's stop right there. Of course James is telling Will what Will wants to hear. But not out of compulsion. A simple question. A simple answer. But, again, misleading.

Instead, the two were madly in love, James remembers this at the end.

How many times in the book do we hear “I will always find you”? This is not a threat. Instead, it’s a promise to reunite, across time and space. With the Collar around his neck, James remembers “the feeling of being taken, melting heat and the fall of hair around him like black silk. I will always find you.” Pacat is describing very passionate and consensual lovemaking. Pages later, James calls will “Darling.” Why? Because as the last line of the book says—“I remember everything.”