As the “Harry Potter” franchise heads into its final lap, the billions generated by films, ancillary and merchandising will eventually dwindle for Warners. As the studio mulls options for new franchise possibilities, recently retired studio topper Alan Horn talked to Variety about what it took to make “Potter” the biggest film franchise ever, and Warner marketing topper Sue Kroll spoke about her approach to marketing the very last pic in the series.

Key to bringing J.K. Rowling’s vision to the bigscreen were British producer David Heyman, who persuaded Warners to acquire the rights to the first four books, and the Warners execs at the time, who invested in what was then far from a sure bet. Though the books were big bestsellers, the kid actors were unknown, and some thought the themes might be too British.

Heyman discovered the first book, “Harry Potter and the Philosopher’s Stone,” a few months after it was published in 1997 and persuaded the studio to acquire the rights for the first four books just before Bob Daly and Terry Semel departed as studio chiefs, replaced by Barry Meyer and Horn. Horn credits studio exec Lionel Wigram as “the unsung hero” who worked through a myriad of details.

“When I got in, there were endless discussions about production design, marketing, music,” Horn recalls. “The marketing was tough because our actors weren’t stars at that point like Tom Cruise or Will Smith, so it was incumbent on us to convince the fans of the books that we would be respectful and deliver on the promise of what they had loved in the books.”

Horn believes it was particularly crucial to consult with Rowling to get the story right. “I always made it clear that she was the most important fan,” he adds.

Daniel Radcliffe, Emma Watson and Rupert Grint were cast in 2000, and the first film, written by Steve Kloves and directed by Chris Columbus, opened in November 2001.

Conversely, the last film, “Harry Potter and the Deathly Hallows: Part 2,” bowing today, is the very definition of a sure bet.

Horn, who departed Warners in April, attended both preems in London and New York.

“As you can imagine, it’s kind of sad in some respects to see it coming to an end, but it’s also been really gratifying to have done eight films and kept the same cast together,” he muses.”We’ve had four amazing directors — Chris Columbus, Alfonso Cuaron, Mike Newell and David Yates — so it’s been kind of like a giant relay race.”

The Potter franchise has been the ultimate in reliability for Warner Bros., with the first seven films grossing $6.4 billion worldwide.

“We are in a wonderful position in that the ‘Harry Potter’ films are truly beloved and widely anticipated by audiences around the world,” acknowledges Kroll.

“The marketing campaign had to engage and inspire audiences and not only prepare them for this final film but remind them of what they’ve loved about Harry Potter over these 10 years,” she says. “To that end, we created a pervasive advertising, publicity and online campaign, complete with a robust content strategy, that reminded audiences everywhere of the true magic that is unique to the world of Harry Potter.”

“The challenges in marketing the epic conclusion to the series had more to do with sustaining an intense level of interest and building on it over these last few months,” says Kroll.

The campaign is aimed at merging the nostalgia for the previous seven films with the unknown as main characters Harry, Ron and Hermione face the evil forces of Lord Voldemort. The marketing presumes that anyone who doesn’t know about Harry, Voldemort and Hogwarts is probably not going to purchase a ticket for “Part 2.”

The marketing campaign revved in late spring with the tagline “It all ends.” Kroll says the phrase is meant to work on several levels. “‘It all ends’ is an emphatic and memorable reminder about not only the end of the film series but the ultimate battle between good and evil,” she says.