Previously best known for middle-grade adventures like Nathaniel Fludd, Beastologist, and Theodosia Throckmorton and the Serpents of Chaos, LaFevers has turned to YA fiction in her dark fantasy trilogy His Fair Assassin, which centers on the mysterious convent of St. Mortain in a gritty, carefully detailed alternate 15th-century Brittany. The final book in the trilogy is Mortal Heart, due out next month. PW spoke with the author from her home in southern California.

How close do you stick to the real history of 15th-century Brittany, and how much historical research did you do for the books?

The really cool thing about this is that 90% of the history in the books is real. You really can’t make some of this stuff up, it’s so wild. Brittany actually was inherited by a 12-year-old girl whose father had promised her hand in marriage to a half dozen different heads of state in return for political favors. He was very wily and did not have any of these things committed to writing; it was all verbal. So when he died unexpectedly and she was left heiress to this very wealthy duchy, all of these people wanted to make good on their claims. So she had to call together her council of advisors to decide which of these men was the best match and would protect her against France, which had been claiming that the Duchy was part of their kingdom and that she couldn’t marry without their permission. She was really determined to maintain Brittany’s independence. Not that many 12 year-old girls could have handled that much pressure.

Once I knew that this was a true thing, I said this is absolutely where I have to set the story. She was a perfect, real-life heroine. All of the politics and the backbiting and betrayal by her Council members, those were all taken directly from history. I actually simplified it a great deal because it was so complicated.

She’s a very believable little girl despite being mature beyond her years.

All of the historical accounts said she was. She was speaking Latin before she was five years old. She was greeting heads of state at four. She was raised with the intention of being the head of state and so she had an exceptional upbringing.

The central fantasy premise of the His Fair Assassin trilogy is that a group of nine ancient Breton gods and goddesses were adopted by Christianity as saints at some time in the past (as in the famous case of St. Bridget of Ireland), but these gods and goddess are still very much active and not at all Christian in their sensibilities. Are St. Mortain and the other gods and goddesses based in real Breton mythology?

Yes and no. Brittany is known for having something like 500 saints, only one of which is actually sanctioned by the Catholic church. They are very liberal with their sainthood-awarding in Brittany. And many of these saints were pulled from Celtic history because Brittany held on to its Celtic roots longer than the other areas surrounding it. So Saint Arduinna, for example, is based on the Celtic goddess Arduinna and Saint Mer becomes in my version the goddess of the sea.

Another reason I was drawn to Brittany was that the country had a very vivid folk figure of death called the Ankou, which drove around in a bone cart and appeared at people’s doorsteps when it was time to die. So I said, wow, that figure has lived so long in their imaginations even until recent years, coupled with the fact that there were so many saints... it just all came together. So some of the saints were pulled from actual folklore and others were made up from whole cloth.

You also had a variation on the Wild Hunt in there called the Hellequin’s hunt. Is that Breton as well?

Actually, it had a French medieval basis. The Wild Hunt appears in so many European cultures so I thought, why not? There were written accounts of priests who had seen the Wild Hunt and claimed it was the tortured souls of the damned, but it had such an interesting combination of factors to it. It seemed perfect for my kind of stitched-together world.

What are some of your influences here, not only historical, but other authors?

One of the books that I read very early on in my teen years that kind of rocked my world was Mary Stewart’s The Crystal Cave and its sequels. That was probably the most influential book on my writing of fantasy with its great story of a historicized Roman Arthur and Merlin. I was so taken by that; she made me believe that these characters actually lived and breathed in history. They’re among my favorite books ever. I think that was the main influence when I started writing. I wanted to write with Stewart’s sense of mythology and history weaving seamlessly together. I think that’s why in all my books, whether they’re middle grade with Theodosia or Nathaniel Fludd, or YA with His Fair Assassin, I’m kind of drawn to those places in history when they almost seem to intersect with what we think of as fantasy, where there’s a chance that fantasy could leak through somehow.

Where did you get the idea of placing assassin nuns in this world?

Doesn’t every troubled 12-year-old need assassin nuns? As I was researching the Middle Ages, I knew that I wanted to write a story of empowerment, but women were so powerless back then. They just didn’t have any avenues to power. Then I was struck by something I read that said that many noble women of the time actually preferred joining a convent to any other societal role open to them because it gave them more freedom and autonomy. Because they were doing God’s work, they were given a little more latitude and they didn’t have to be beholden to individual males who were in charge of their lives. Also, because they were serving saints they were kind of outsiders. And of course this led to the question of who these nine saints would be that they were serving? My nun characters are serving Death, which leads to all sorts of delicious, crunchy moral questions.

Ismae, the heroine of Grave Mercy, Sybella, the protagonist of Dark Triumph, and now Annith in Mortal Heart, are all powerful and likeable protagonists, though each has a variety of very dark secrets. How do you maintain a balance, creating characters with whom your readers will connect, even empathize, while at the same time making all three of them scary as hell?

Oh now, see, that’s funny because I don’t see any of them as scary! Well, okay, maybe Sybella, but that was because she flirted with madness and walked the razor’s edge of despair. The only thing holding her to this world at times was her love for her sisters and the knowledge that if she was gone there would be no one to look after their interests. Which, in turn, I think is the key to making her character both empathetic and relatable – she had utterly valid reasons for being both so despairing and so deadly, reasons readers were able to understand and sympathize with once they knew them.

There was a similar dynamic with Ismae; I think once readers knew her past, they understood why she would so cheerfully take up her duties and commit to them so fervently. Those duties had taken her from abused and powerless to a force to be reckoned with – and who wouldn’t embrace that?

With Annith, I think the trick was to show that as deadly as she was, she also had strong moral convictions and a sense of right and wrong that steered her.

Another important factor is that the scary, fierce part of these characters was only one of many facets and most often, a coping mechanism they had developed through years of hardship. So, I think the important thing is the character’s backstory and what motivates them to act the way they do. If the reasons are ones we can empathize with – or at least understand – then it becomes possible to root for the characters, no matter how scary they might seem.

Ismae and Sybella each find romance in their respective novels with men who are truly worthy of them, but Annith, who sees herself as the most human of the three young women, falls in love with Balthazaar, who is quickly revealed as someone decidedly more than human and in fact one of the damned? Do they really have a future together?

Yes, they absolutely have a future together, but it will not be an easy one or one with as clear a path as Ismae and Sybella have. They will have to forge their own way every step. Part of this is that Annith is still struggling so much with her own sense of being flawed and realizing that something is inherently wrong with her. There’s a relief in finding someone who’s both damned and attractive. Somehow it makes being damned less hideous and helps her look more kindly on her own flaws and possible damnation. It challenges her and her world view in a way that falling in love with a noble human would not have. And of course as an author you always want to crack your characters wide open and force them to make hard choices and put them through this crucible so that they can make transformative changes in their lives. At least that’s what I like to do.

Is Mortal Heart a true conclusion to the series or are we likely to see more His Fair Assassin adventures?

We are likely to see more in the His Fair Assassin world. The stories centered on the Duchess of Brittany are pretty well concluded at this point. She does have some interesting things that happen in her future, but whether they will serve as a dramatic impetus for more books, I’m not sure. But there are a lot of places in Breton history that are ripe for exploring. The more I wrote, the more I kept thinking about how all of these different girls would end up at this convent, and how they would get there, and what their life circumstances would be and how that would shape them. We know what the convent was like when Ismae, Sybella and Annith were there, but what was it like 100 or 150 years earlier during different circumstances?

So there’s lots of ideas for me to play with and I will definitely be adding more. I thought about doing a book set back in the time when Mortain became a saint but then you’re talking about the year 1000. That’s a little bit harder to pull off in terms of verisimilitude. It would be fascinating; I just haven’t figured out how to make that happen yet.

If a bookseller asked you how you would recommend that she handsell Mortal Heart or His Fair Assassin generally, what would you suggest?

To be honest, I’ve heard from booksellers and librarians that almost universally all they have to say is “teen assassin nuns in medieval France” and it’s sold. Sometimes all they have to say is “assassin nuns” and it’s sold. That kind of does it. And it’s funny because I really did not sit down to write a book about assassin nuns. I actually thought of them primarily as Death’s handmaidens and it wasn’t until the book was finished and I was talking it over with my agent and my editor that it occurred to me that they’re basically assassin nuns. I didn’t sit down with this really kooky concept; it just sort of evolved out of the world I had created and it has ended up being a huge attention getter. The two words, “assassin nun,” are so far apart, almost oxymoronic. They’re so contradictory; how can you even have those two words so close together? They give the imagination a lot of room to wiggle.

What are you working on now?

Between the Theodosia books and the Nathaniel Fludd books and His Fair Assassin, I’ve been on deadlines since 2009, and I kind of said, can I please have a break? These were really big books to write in a short amount of time. With Grave Mercy, I worked on that off and on over six years between my other contracts with the shorter middle-grade books. It really took a lot of time to build the world and create the characters and go over things in my mind. Obviously with Dark Triumph and Mortal Heart I had much less time. It was a bit of a forced march for the creative part of my brain, so I’m taking a few months off. It’s kind of nice to enjoy the publication of Mortal Heart without being on deadline.

Mortal Heart by Robin LaFevers. HMH, $17.99 Nov. ISBN 978-0-547-62840-0