If you’ve spent any time at all lost in Deborah Harkness’s All Souls trilogy, you know the feeling. One minute you’re tracking Diana Bishop’s journey through the Bodleian Library, and the next, you’re gut-punched by a character who feels like the beating heart of the entire series. I'm talking about Jack Discovery of Witches fans usually refer to as "Jack Blackfriars," though his real name is Jack Callow. He’s the street urchin who shouldn't have mattered to the plot but ended up becoming the literal anchor for Matthew and Diana’s humanity.
He’s complicated. He’s a vampire with a "blood rage" problem that would make even the most seasoned de Clermont flinch. Honestly, his story is probably the most tragic arc in the whole franchise.
The Orphan Who Changed Everything
Most people meet Jack in Shadow of Night. It’s 1590s London. Matthew and Diana are playing "hide from the congregation" while trying to find a teacher for Diana's magic. Enter Jack. He’s a starving, filthy, incredibly resilient pickpocket. He tries to rob them. Most vampires in that era would have just drained him and left him in a gutter. But Matthew—in a rare moment of Elizabethan-era soft-heartedness—decides to take him in.
It’s a weird dynamic. Diana immediately treats him like a son. Matthew treats him like a ward. Jack becomes the first real piece of evidence that Diana and Matthew aren't just a couple; they're a family. It’s small details that get you. The way Diana teaches him to read. The way he clings to them. He’s the "human" element in a world of monsters.
But things go sideways. They always do. When Matthew and Diana have to return to the present day, they can't take him. They leave him behind with the Hubbards, thinking he’ll live a full, human life. They were wrong.
What Happened to Jack? The Transformation You Didn’t See Coming
This is where the Jack Discovery of Witches fans might get confused if they haven't read the books or finished the TV series. Jack didn't die of old age in the 1600s. Instead, he was turned into a vampire.
Father Hubbard, the "King of the London Underground" and a vampire himself, turned Jack. Why? Because Jack was dying of the plague. Hubbard didn't do it out of malice; he did it to save the boy Diana loved. But there’s a catch. Jack was born with the "blood rage" gene.
If you aren't familiar with the lore, blood rage is basically a genetic mutation in the de Clermont line (and those sired by them) that makes vampires lose all control. They become killing machines. It’s the reason the Congregation wants to wipe out the de Clermonts. Jack becomes the physical manifestation of Matthew's greatest fear. He is a blood-raged vampire, sired through a convoluted lineage, living in the shadows for centuries, waiting for his "parents" to return.
The Benjamin Factor
We have to talk about Benjamin Fuchs. If Jack is the heart of the story, Benjamin is the rot. Benjamin is Matthew’s disowned son, and he’s the one who finds Jack in the 20th century. He uses Jack. He tortures him. He trains him to be a weapon.
Imagine being Jack. You’re abandoned by your parents in 1591. You’re turned into a monster. You spend 400 years being manipulated by a psychopath who hates your father. When Jack finally reunites with Matthew and Diana in The Book of Life, he isn't a sweet little boy anymore. He’s a killer. But—and this is the part that makes me cry every time—he still remembers the "Goody Alsop" lessons. He still loves Diana.
Jack’s Return in the Modern Day
When Jack reappears in the modern timeline, it throws a wrench into everything. The Congregation is already breathing down Matthew’s neck about his relationship with a weaver. Now, there’s a blood-raged vampire running around London, and he’s claiming to be Matthew’s son.
It’s a political nightmare.
In the TV show, Jack is played by Toby Regbo. He nails that jittery, traumatized, yet fiercely loyal energy. You see the conflict in his eyes. He wants to be "good," but the blood rage is a physical itch he can’t always scratch.
The most pivotal moment for Jack Discovery of Witches lore is when Matthew has to decide whether to cull him. According to de Clermont law, blood-raged vampires must be killed. It’s Matthew’s job to do it. The tension in those chapters is thick enough to cut with a dagger. Does a father kill his son to protect a secret? Or does he find another way?
Why Jack Callow Matters More Than You Think
Jack is the proof that the Covenant is broken. The Covenant—the law that says species shouldn't mix—is based on the idea that mixing leads to instability. But Jack shows that "family" isn't about species. He’s a vampire, raised by a witch and a vampire, who was originally a human. He’s a mess of identities.
But he’s also the key to the DNA research Marcus and Christopher Roberts are doing. By studying Jack, they realize that blood rage isn't just a "curse." It’s a specific genetic trait that appears when vampire and weaver DNA interact over generations. Jack isn't a mistake. He’s a biological revelation.
The Differences Between the Book and the Show
Honestly, the show handles Jack’s trauma quite well, but the books give you the internal monologue that makes his pain feel more visceral. In the books, Jack’s artistic talent is a huge deal. He paints. He remembers. In the show, his presence feels a bit more like a ticking time bomb for the plot.
- The Portraits: In the source material, Jack keeps the portraits of Matthew and Diana for centuries. It’s his only link to his past.
- The Hubbard Connection: The relationship between Jack and Father Hubbard is much more developed in the novels. Hubbard is a "vampire priest" who treats his flock like a literal congregation.
- The Final Battle: Jack plays a crucial role in the confrontation with Benjamin. It’s his chance for revenge, but more importantly, it's his chance for redemption.
What Most People Get Wrong About Jack
A lot of casual viewers think Jack is a villain when he first shows up in the modern day. He’s responsible for some pretty grisly deaths in London. But he’s not a villain. He’s a victim of his biology and Benjamin’s grooming.
You’ve got to look at it through the lens of mental health. Blood rage is often used as a metaphor for neurodivergence or uncontrollable trauma responses. Jack isn't "evil." He’s overwhelmed. When Diana uses her "weaving" to help him, she’s not just using magic; she’s providing the structure and boundaries he never had.
Actionable Steps for Fans of the All Souls World
If you’re obsessed with Jack’s journey and the wider Discovery of Witches universe, here is how you can dive deeper into the lore and the history behind his character:
- Read "Time’s Convert": This is Deborah Harkness’s fourth book in the series. While it focuses heavily on Marcus Whitmore, it provides essential context for how the de Clermonts deal with their "fledglings" and the history of blood rage.
- Explore the Elizabethan History: Jack’s origins are rooted in the poverty of 16th-century London. Researching the "Blackfriars" district of the 1590s will give you a much better appreciation for why Jack was so desperate when Matthew found him.
- Study the Genetics of the Series: If you’re a science nerd, look into the "Mendelian inheritance" theories fans have written regarding blood rage. It explains why Jack has the condition while others don't.
- Rewatch Season 3, Episode 5: This is where the emotional weight of Jack’s return hits its peak. Pay attention to the blocking in the scenes between Matthew and Jack—the distance between them tells the whole story.
Jack Callow isn't just a side character. He is the bridge between the past and the future of the de Clermont family. He’s a reminder that even in a world of immortal creatures and powerful magic, the most powerful force is still the bond between a parent and a child, no matter how "broken" that child might seem.