A high-voltage shock zapped my arm. A bolt of lightning shot from my demon mark and scorched the ceiling. I shrieked and tried to hurl the book away. But it stuck to my hand like it was glued there.

I jumped up. Mab leaned away from me in her chair, her features round with surprise. I shook my hand, trying to shake the book away. My fingers gripped it like a vise, so hard they hurt. I couldn’t let go.

Something in my peripheral vision glowed red. That lightning bolt—was there a fire? I turned. No flames, but the place where my father fell glowed with red, misty light. The light brightened until it blazed as vivid as a sunset at its peak, the red shot through with streaks of purple and green—Dad’s colors. Then, like a sunset but faster, the light faded. Tears welled as I watched the glow disappear. When it was gone, the tears spilled over, wetting my cheeks.

My father had been dead for ten years. Yet he’d just told me he wanted me to do this.

I still held the book, although my fingers had relaxed their death grip. A residual tingle and odors of sulfur and charred flesh lingered, but otherwise I felt normal. I was standing in my aunt’s library, holding a book, something I’d done hundreds of times.

I sat down and turned to the first page.

IT WOULD BE IMPOSSIBLE TO RECORD WHAT THE BOOK ACTUALLY said. It wasn’t like normal reading, where you run your eyes over the page, noticing how letters form words and words are arranged into sentences. Instead, I stared at the jumble of odd-shaped letters until my vision blurred. And then—sometimes sooner, sometimes later—a block of knowledge appeared in my mind.

Later, I tried to decipher the language by matching bits of that knowledge to groups of letters and symbols, but it always fell apart. Understanding receded and the letters closed ranks, moving around on the page to hide what I’d thought were divisions between words.

Mab had said it was the language of Hell. She said the book would try to trick me. To someone who got straight Cs in English, this wasn’t great news.

The book’s first chapter, and the only part I could read that day, was a history: the origins of the Cerddorion and demons we oppose. In that sense, it was like the Cerddorion history I’d studied at the start of my training, including the Mabinogion, a group of medieval Welsh stories, poems, and myths. Dad was a scholar of Old Welsh literature, and he loved the Mabinogion, but I’d never been all that interested. Mostly, it narrated endless battles and knights performing impossible tasks. The little bit about the origins of my race was written by a norm who clearly didn’t approve of magic or believe in shapeshifting. For starters, the Mabinogion demoted Ceridwen, the mother of my race, from a goddess to a witch. I’ve known some perfectly nice witches, but a goddess is a goddess.

The Book of Utter Darkness told the story from the demons’ point of view. It began like other histories, with the birth of Ceridwen’s first child, a son. Traditionally, Ceridwen’s baby was described as ugly, even hideous, but not here. According to The Book of Utter Darkness, the child had a noble bearing. He was also hungry, insatiably so. The moment his mother stopped feeding him, he screamed and howled. And each time he opened his mouth, the spirit of his hunger emerged—the Morfran. And so the essence of demons was created.

That’s why Morfran had sounded familiar: I’d seen the word in the Mabinogion. There, though, “Morfran” was a nickname for Ceridwen’s baby, who was so ugly his own mother called him “Great Crow.” Maybe the author of that account didn’t want to believe a newborn baby could be the source of all demons.

I turned the page to a hand-drawn illustration: A naked infant lay on the ground, arms and legs waving. From his wide-open mouth emerged a huge black bird—the Morfran, the great crow, created by the child’s hunger. I studied the picture for several minutes, then stared at the incomprehensible words until the story continued in my mind.

The constant feedings exhausted Ceridwen as the infant literally drained her life. She named the baby Avagddu—the name means “utter darkness”—because each time she fed him, he suckled so hard and so long the world began to go black. Unwilling to let the child deplete her life force, she gathered herbs to make a potion that would allow the baby to change its shape. That way, the hungry infant could become a calf, goat, sheep, or dog and suckle from other animals, letting Ceridwen regain her strength. She hired a shepherd boy, Gwion, to stir the potion as it boiled in a cauldron over an open fire. Gwion’s instructions were to keep the fire going and stir without ceasing until only three drops of potion remained. Those three drops would hold the concentrated magic to turn Avagddu into a shapeshifter.

As the shepherd boy stirred, the exhausted Ceridwen stuffed rags in her ears and went to sleep. She slept for weeks without waking. As Avagddu cried, more and more Morfran emerged. The cauldron absorbed some of it, and some possessed Gwion, already a greedy child, who began to hunger for the potion. At the crucial moment, Gwion scooped up the last three drops and, hot as they were, swallowed them. The overheated cauldron shattered, and Avagddu’s enraged howls reached a new pitch.

The Book of Utter Darkness depicted Gwion differently from other sources. Here, he was grasping and sneaky—a prime target for Morfran possession. In other histories, the boy started off as either ambitious or foolish, but he gained great wisdom from Ceridwen’s potion.

Back to the book. The shattering of the cauldron awoke Ceridwen, who saw what had happened. Furious at Gwion for consuming the magical drops, she chased the boy. As he ran, he changed into a rabbit to gain speed. Ceridwen shifted into a greyhound. When she gained on him, he leapt into a river and transformed into a fish; Ceridwen became an otter to hunt him there. The chase continued like that: Gwion shifting as he tried to escape and Ceridwen right behind him, her shifts always erasing his advantage. At last, when Gwion could run no more, he hid in a barn, where he dived into a storage bin and became a grain of wheat, hiding in a huge pile of other grains. Ceridwen changed into a black hen and pecked inside the bin. She, too, had a great capacity for hunger, and she gobbled up every grain—including Gwion.

The book implied this was a suitable end to Gwion: A greedy, treacherous boy succumbed to his hunger and landed in another’s stomach. But, as anyone who’d read the Mabinogion knew, it wasn’t his end. I turned the page. The story was appearing fast in my mind, as fast as if I were reading it. The next section related a scene new to me.

Ceridwen returned home to find Avagddu quiet, playing with a fragment of the shattered cauldron, cooing and smiling. Ceridwen knew the cauldron had absorbed much of the Morfran and become saturated with poison as the potion boiled. She tried to snatch the shard away from her son, but the hot iron burned her hand. Avagddu laughed at her pain, then went back to babbling at the fragment. Ceridwen watched him. He looked like he was having a conversation with the shard. Although the iron had burned her, the child seemed unharmed. And for the first time in months, the baby wasn’t howling. She let Avagddu keep his plaything.

Nine months later, Ceridwen gave birth to Gwion anew. The baby was perfect, and she named him Taliesin—“radiant brow”—to honor his beauty. Ceridwen favored the new baby, igniting a fierce hatred in Avagddu. The older boy spent his time huddled with his precious cauldron fragments. Ceridwen still couldn’t touch them without burning herself.

Avagddu drew dark magic from the fragments and transformed himself into a poisonous mist. The mist spread over Taliesin in his cradle, and the baby cried out for their mother’s help. Ceridwen came running and fanned Avagddu away. The older boy approached Taliesin as a snake, a beautiful butterfly, and a rain shower—all poisonous. Each time, Ceridwen rescued her younger son. But she realized Avagddu would never quit trying to kill his brother, so she sent Taliesin away to be raised by a human family. Frustrated that his hated brother was beyond his reach, Avagddu swore eternal enmity against Taliesin and the humans who fostered him.

And so three races were born with these events. Taliesin’s descendants were the Cerddorion. Avagddu fathered a race of demi-demons, the Meibion Avagddu, or Sons of Utter Darkness. And each searing, poisonous, iron-hard fragment of the shattered cauldron became a Hellion. The largest of them, Difethwr, remained Avagddu’s companion and chief retainer throughout his life.

I LOOKED OVER AT MAB, WHO STARED INTO THE FLAMES dancing in the fireplace. I’d been so deeply absorbed in the book, I didn’t notice her build a fire. Her eyes met mine.

“Are you all right, child?”

“Yes.” And I was, despite a slight ache behind my eyes from staring at the pages as the book whispered its story in my mind. The story had been familiar, yet different, with its emphasis on Avagddu and demons. In the Mabinogion and the Cerddorion histories I’d read, the whole point of the story was Taliesin’s birth. To the norms, Taliesin was a hero and the greatest Welsh bard. To the Cerddorion, he was the first member of a glorious race of shapeshifters. But in The Book of Utter Darkness, Taliesin was the enemy.

“This is the first I’ve heard of the Meibion Avagddu,” I said. “What are demi-demons?”

“They’re half demon, just as you are half human. Their shapeshifting powers are different. They have a human form, which enables them to move about in daylight. But they cannot turn into other creatures, as we can. Rather, they have shadow demons. That means they can assume their demonic form from twilight until dawn. But even in full daylight, the demonic half shadows them, following them in the demon plane.”

She held out her hand for the book. I was happy to pass it to her. As she took it, a cool, soothing sensation spread over my demon mark, like aloe vera on sunburn.

“The Meibion Avagddu are a depleted race,” Mab continued, holding the book in her lap and smoothing a hand across its pale cover. “Their females are barren. The males sometimes mate with human women, but two-thirds of the children from such couplings are stillborn. Of those born alive, half die in infancy.”

“Is that why I’ve never run into any?” Then I realized. I had run into one—who’d said we shared a common ancestor although he wasn’t Cerddorion. “Pryce is a demi-demon.”