“Go on.”

“I was able to isolate the precise frequency: the flatted or diminished fifth.”

I had less than a month of music theory. “What’s the flatted fifth?”

Dancer says, “Mi contra fa est diabolus in musica—where the mi and fa don’t refer to the third and fourth notes of the musical scale but to the medieval principle of overlapping hexachords.”

I say impatiently, “Clarify.”

“Also known as Satan’s music, or the Devil’s tritone, it’s an interval spanning three whole tones, such as C up to F# or F# up to C, the inverted tritone. It’s used in sirens, can be found in the hymn ‘A Mighty Fortress Is Our God,’ Metallica’s ‘For Whom the Bell Tolls,’ ‘Purple Haze’ by Jimi Hendrix, ‘Black Sabbath’ by Black Sabbath, Wagner’s Götterdämmerung, the Dante Sonata by Liszt, Beethoven’s—”

“We get the picture. Get on with it,” Barrons growls.

“Mathematically speaking, harmonies are created by notes sounding together in proportion to one another that can be expressed in numbers. The Devil’s tritone is commonly assigned the ratio of 64/45 or 45/32, depending on the musical context … And your eyes are glazing and I haven’t even gotten started,” Dancer says. “Okay, then, it’s jarring, disconcerting, some even consider it depressing. There’s a lot of controversy about whether or not ecclesiastical sorts banned it in medieval times out of fear it could summon the devil, him—” He breaks off and grins at me. “—or herself. How’s that for laymen speak? Personally I find it challenging, invigorating—”

“Again with the we-don’t-give-a-fuck,” Ryodan says. “Tell her what you told us.”

The grin fades. “Like music, all matter is composed of frequencies. Where the Hoar Frost King took his ‘bites’ of melody from the world, it completely consumed that frequency.”

“What are you saying? We have no flatted fifths left?”

He gives me a look like I have two heads. Math and physics have never been my strong suits.

I guess again. “It’s quieter in the places he iced?”

Dancer says, “In a sense. Cosmically. And that’s only part of the problem.”

“What’s the real world application?” I growl. Nobody likes feeling dumb.

“I’m getting there. I had a hunch. I’ve been going back to the scenes every day. I didn’t find what I was looking for until a few days ago and have been observing it since, taking measurements, projecting and speculating on the potential ramifications of—” He breaks off and looks at Ryodan. “I think we better show her. Telling her doesn’t seem to be working. I thought you said she was smart.”

“I took Barrons’s word for it.”

“Apparently he was misinformed,” Dancer says.

I have the beginnings of a headache. “Oh, shut up both of you, and just show me what you’re talking about.”

“I think the church is the closest spot where she can get a good look,” Dancer says. “The one outside Chester’s is still forming.”

Ryodan looks pissed. “I’ve got one closer.” Whatever it is, and wherever, he’s not at all happy about it.

I follow the three of them to the door of one of the many sleekly concealed elevators in the club.

Because there isn’t enough room for my volt of vultures to maintain their distance from the men when we step inside, I get a respite. I hear thumps as they settle on the roof of the compartment.

We ride down. And down. Through the walls of the elevator, I watch the levels of the club whiz by as we descend into the chrome and glass belly of the beast. Like the city hidden beneath the abbey, the private part of Chester’s is enormous. There’s no way they built it all recently. I wonder if it’s been standing as long as or longer than the sidhe-seers’ hallowed enclave, and if so, where they got the building materials back then.

We continue dropping for half a mile or more. I can feel tons and tons of earth around and above me and shiver. I’ve always hated being underground but my interment in Mallucé’s lair beneath the Burren escalated dislike to near claustrophobia. I can barely breathe down here.

As we begin to slow, Ryodan says, “Do not exit until I do. Then follow me, remaining behind me at all times.”

The compartment settles and the door swishes open.

I move into the dark, silent corridor behind his broad back.

The air is chillingly cold.

It’s so dark that I instinctively open my sidhe-seer senses to scan for the unique Shade frequency—a trick I perfected last month when I discovered a ship down near the docks where several of the vampiric Unseelie had holed up—and instantly my head explodes with pain.

I fall to my knees, clutching my skull with both hands, crying out.

I haven’t felt pain like this since the night I went to meet Christian at Trinity College. I made it only a few blocks before the Sinsar Dubh reduced me to a gibbering, drooling mess in a gutter in Temple Bar, crushed by the agony it was inflicting.

Spikes pound through my brain. My stomach cramps and my spine becomes a red-hot poker impaling my body.

Pain fills me until I’m nothing but a single, giant exposed nerve alternately being raked over coals, then diced and iced, before getting seared again.

Barrons has me then, his arms strong, sheltering. “What the fuck, Mac?” he growls. “What’s happening?”