In The Tower of Nero, the fifth and final book in the Trials of Apollo series, Rick Riordan brings his epic saga, which began in 2005 with The Lightning Thief, to a conclusion—at least for now. Apollo—trapped in the form of a teenage boy and mostly bereft of his godly powers—must face off against the greatest evils of the Roman Empire in order to save the world. With 549 weeks and counting on the New York Times bestseller list, Percy Jackson and the Olympians has proven massively popular over the last 15 years. We spoke with Riordan about the end of this journey, and what to expect next.

After 15 books and assorted related projects, you’re apparently wrapping up the Percyverse. Is The Tower of Nero really the final chapter for our heroes?

Well, it’s the last book of the last series. I don’t anticipate doing any more epic five-book series. I’ve pretty much gone that route and done what I wanted to. I’m not closing the door on telling smaller stories or one-off novels, but I don’t see any more grand adventures. Honestly, I’ve done Greek and Roman mythology pretty thoroughly. There aren’t a whole lot of corners I haven’t dusted out looking for new and interesting monsters and villains.

My operating strategy from the beginning was to not hold back, to make every book as much of a thrill ride as I can. The fact that I’ve been able to get 15 books out of this tells you a lot about how deep Greek and Roman mythology is, and how much there is to it. But the ending of Tower of Nero leaves things in a pretty solid place. You get a good sense of where all the characters you’ve met in the last 15 books are going and how their lives are unfolding. I don’t want to seal everyone’s fate forever; I’m a big nonbeliever in epilogues, and I want to leave a little room for the readers to imagine what happens next.

So you chose to end things with Percy finally heading off to college as a symbolic moment?

He gets to road trip off into the sunset and into a brand-new chapter of his life, yes. It was time to finally, after all these years, let him turn 18 and become an adult. He may have more adventures, but he’s in a good place and living his life.

How does it feel to end things after so much has happened?

It feels really good. It’s bittersweet, of course, because I’ve been living with these characters for so long and have watched them grow up in tandem with my own children. But my own are 26 and 21, and have gone through the college experience, and now it’s Percy’s turn. This has been such a huge part of my life for so long, and the characters really are like family members. But I feel lucky and fortunate that I’ve gotten the chance to do this and that so many readers have followed the story. It’s a lot to ask readers to stick with a story for 15 books, but they have. It’s been a pretty incredible ride.

You started writing Percy Jackson for your son, right?

That’s right. My older son, Haley, has ADHD and dyslexia, and was the impetus for Percy Jackson when he was eight or nine years old. I started telling him a bedtime story, and everything came from that. He needed a story that would tell him that it was okay, that seeing the world differently, processing information differently is okay, and can be a sign of strength. And my son just got his master’s degree in higher education with a certificate in learning disabilities. He took that whole life experience and turned it around, and now wants to help other kids.

How else have things changed over the past 15 years for you? Has your process evolved at all?

It’s easier to say what hasn’t changed. When I started, I was a middle school teacher, and my wife was at home with the kids. We were just struggling to get by. After Percy took off, it became a whole different thing. The success didn’t happen overnight; after about five years we looked back and said “Whoa, what’s happened here?” But it’s been an incredible ride. I’ve learned more about myself as a writer. Writing books never gets easy. I’m on book 31, and it’s still hard. I’ve learned that outlining really makes a difference. And it’s good for me to mix things up a bit. For instance, Percy was a first-person narrator, while I told The Heroes of Olympus from seven different third-person points of view. And then for The Trials of Apollo, I went with a first-person narrator again, but instead of a 12-year-old boy, it’s a 4,000-year-old god, who’s totally full of himself.

And of course I try to write like I taught in the classroom. I try to educate without making it seem like I’m lecturing, to make it fun and relevant, so kids will enjoy it without realizing they’re learning along the way. They learn mythology and folklore in a way that’s easy to relate to.

You’ve written several other series that stand alone but share the same world. Do you consider the Kane Chronicles and Magnus Chase series to be part of the Percyverse?

It depends on how you look at it. From an intellectual property point of view, my lawyers would love for me to say they’re separate. From a narrative point of view, there have been crossovers. The Kanes have met Percy and Annabeth in a series of short stories. Magnus is Annabeth’s cousin and so Percy and Annabeth have both made cameos in that series. They exist in the same universe, but there’s room for a lot of mythologies in the same world. There always have been. The Romans and Greeks certainly understood that the Egyptians had a completely separate pantheon, and so did the Celts and the Persians. It wasn’t a contest between them. The truth depended on where you were, and there was a lot of overlap.

You’ve mentioned that one of your potential upcoming projects is Celtic-related, and you recently pursued a master’s degree in Gaelic literature. What can you tell us about that?

One of the most interesting mythologies I haven’t yet explored, that I feel I could do so with a certain degree of authenticity, is Celtic. It’s my own ancestral heritage, and it’s fascinating stuff. It’s really quite different and hard to quantify the mythology from its beginnings simply because the Druids didn’t write anything down. We’re left to rely upon archaeology and accounts from outsiders like the Greeks and Romans, or much later medieval accounts. So it was a great treat to get my master’s degree with the University College in Cork. They were fantastic. I did my thesis on the god Lugh and the Tuatha Dé Danann. I haven’t decided exactly what I want to do yet, but I have more ideas than I can possibly use. It’ll probably be a standalone story, until we see how it’s received and how I feel about it. I don’t anticipate another big epic series quite yet. I don’t even know if I want to do modern-day or something set in ancient times.

When asked about the possibility of telling stories from cultures whose identity you didn’t share, you partnered with Disney to create the Rick Riordan Presents imprint. What can you tell us about that?

The Rick Riordan Presents imprint has been the most amazing, rewarding experience, with so many fantastic authors and stories. I love to talk about books I enjoy, and this gives me a chance to do that, and share them with a huge audience. I feel comfortable saying that if you like my books, if you like that blend of humor and action and folklore and mythology, you’ll love these authors too. I feel that it’s so much better to let people tell their own stories. For instance, I love Chinese mythology, but I’m by no means an expert. They’re not the stories I grew up with. So it’s better to find an author who’s from that background and knows that perspective and is really integrated into those stories, and have them take on their own traditions and heritage. And we’ve gotten amazing support for the imprint from just about everybody, and so it’s succeeded beyond my wildest expectations. At this point, these authors are like a found family, and a fabulous group to work with.

As the Percyverse developed, you’ve introduced a number of LGBTQ characters, despite critics who claim middle grade is no place for this sort of identity exploration. What are your thoughts on this?

Whenever I’m writing, I try to think of my own students and what they’d want to read, what’s important and relevant to them. No matter who you are, middle school is a very tough time. It’s all about identity exploration, whatever that means for each individual. You’re trying to figure out who you are, where you stand with family, with friends, with authority figures. You’re changing physically, emotionally, developmentally—there’s no way you’re not changing when in middle school. If that’s not a perfect time to talk about exploring identity, I don’t know when it is. It’s a critical time we need to acknowledge for all kids. The best message we can give them is that you’re not alone. You’re seen and supported no matter what you’re going through. To pretend that a huge cross section of our kids doesn’t exist, that doesn’t serve anybody.

New live action adaptations for Percy Jackson and the Kane Chronicles have been announced. Can you share anything about these?

It’s kind of funny. I’d decided last October to go into a semi-retirement. I’d just finished my master’s degree, and was planning to go for my doctorate in Celtic studies. I’d been accepted to the Ph.D program at Harvard, and was ready to spend the next six years as a student while the writing took second place. And Hollywood called. There’d been changes in the companies who owned the rights to the properties, and there were new openings for things to happen. Last time, my wife, Becky, and I, had a very poor experience. We weren’t involved, we weren’t consulted or listened to. It left a bitter taste in our mouths. A year ago, if you’d asked me if I’d ever work with Hollywood again, I’d have laughed at you. But we thought about it, and I decided to bite the bullet. I felt I owed it to my fans to give it another shot, after the way the feature films disappointed everyone. I declined my admission to Harvard, and instead of learning ancient Celtic languages, I started learning the language of Hollywood. This time, we’re fully involved, every step of the way, as full producers on the Percy Jackson television series on Disney+, and for the Kane Chronicles movies on Netflix. I’m guardedly optimistic that we’ll see some high-quality adaptations out of this.

What else are you working on?

Currently, I’m working on the stuff for Hollywood, like the pilot episode for the Percy Jackson series. That’s taking most of my bandwidth. I have a good team helping me, but adapting for television is still difficult. I have a few other irons in the fire, like four standalone novels I’m tinkering with simultaneously, but nothing I can really speak of. We’ll see what comes of it eventually.

The Tower of Nero (Trials of Apollo, Book Five) by Rick Riordan. Disney-Hyperion, $19.99 Oct. 6 ISBN 978-1-4847-4645-5