The horror genre as a whole suffered a severe blow Sunday afternoon with the passing of a legend: George A. Romero. The famed director of Night of the Living Dead was inarguably the most important single person in the history of the zombie subgenre of horror, giving birth to a modern conception of the living dead that has been almost as durable as its namesake for the last 50 years. To say that Romero was influential is like saying that Bram Stoker impacted how we perceive vampires.

He reportedly passed away after “a brief but aggressive battle with lung cancer,” with family at his side, acting every bit the film director: Listening to the score of the classic 1952 John Ford/John Wayne feature The Quiet Man. Already, the remembrances are pouring in via Twitter, including collaborators such as Stephen King and genre icons such as Bruce Campbell.

Sad to hear my favorite collaborator—and good old friend—George Romero has died. George, there will never be another like you. — Stephen King (@StephenKing) July 16, 2017

Hailed as a godfather of not only zombie cinema but independent cinema itself, Romero’s 1968 debut feature changed everything in the U.S. horror genre. Made for a pittance, it was a surprise, smash hit on the drive-in circuit, where it immediately developed an infamous reputation. Pushing the boundaries of overt gore and violence in the black-and-white horror medium, it was instrumental in ushering in a new age of more extreme, exploitative horror, which proliferated in the ‘70s grindhouses in the next decade. Although NOTLD may seem quaint today, existing as one of the most notable films in the public domain, it was a singular horror experience that was seared into the memories of an entire generation of future directors.

At the same time, NOTLD was also the film that codified the “rules” of the modern zombie—a word that coincidentally never appears in the film, where they’re primarily called “ghouls.” Prior to Romero, most instances of zombie fiction were so-called “voodoo zombies,” of the sort that had been seen since 1932’s White Zombie with Bela Lugosi. These proto-zombies weren’t necessarily the truly reanimated dead, but rather living people who had been put under the psychological or chemical control of a ringleader. Romero’s ghouls, on the other hand, truly were the living, shambling dead: Reanimated corpses that appear without warning the world over, hungry for the flesh of the living. In creating them, he drew on a few clear inspirations, such as the 1966 Hammer feature Plague of the Zombies, as well as the classic Vincent Price vehicle The Last Man on Earth, which similarly featured a man in a house besieged by ghouls. But everything else, Romero crafted from the whole cloth.

It’s impossible to overstate how influential this very concept was for modern horror. It was a freshly urbanized brand of terror; a creeping unease about your neighbors and strangers that was a natural bedfellow to Cold War-era suspicion. Rather than the gothic manors of Dracula or The Old Dark House, or the remote, exotic locations of tropical monster films such as Creature From the Black Lagoon, the zombies of Romero’s nightmare showed up on your very doorstep, or in your own backyard. In that sense, John Carpenter only needed to look toward Romero for insight when bringing slashers to suburbia in Halloween. Perhaps that’s what he was thinking as well, when he tweeted the following:

George Romero was a great director, the father of modern horror movies. He was my friend and I will miss him. Rest in peace, George. — John Carpenter (@TheHorrorMaster) July 16, 2017

Despite that, Romero seemed genuinely unsuspecting of the prodigious monster he was creating at the time. In an interview with NPR in 2014, he said he “never expected it.”

“All I did was take [zombies] out of ‘exotica and I made them the neighbors,” he said then. “I thought there’s nothing scarier than the neighbors!”

Nevertheless, Romero’s zombies came to define the very term, to the point that any deviations from the formula of the “Romero ghoul” are seen as novel or remarkable simply for the fact that they deviate—see the brain-hungry zombies of former Romero partner John Russo in Return of the Living Dead, or the sprinters of Zack Snyder’s 2004 Dawn of the Dead remake, which presaged similar “infected” in 28 Days Later. How influential were Romero’s conventions? So all-encompassing that it took 36 years for someone to make those monsters run rather than walk. That’s the kind of respect we’re talking about here.

In the years that followed, Romero brought a similarly raw, residential feel to vampire fiction in 1978’s underrated, underseen Martin before returning to zombies with the next two entries in his original zombie trilogy, Dawn of the Dead and Day of the Dead. First teaming with legendary special effects technician Tom Savini, who brought a new level of crimson blood splatter to Dawn of the Dead, Romero used the cloak of grindhouse aesthetics to advance his own brand of social satire and critique of consumer culture and human corruptibility. Along the way he also advanced the mythology and capabilities of the ghouls themselves, suggesting a capability for learning and intelligence in both Day of the Dead and Land of the Dead.

Outside of the zombie works, one would be remiss to not note Romero’s work with Stephen King on Creepshow and Creepshow 2, two of the finest horror anthologies of the ‘80s, in a decade that was rife with them. Who could forget the seminal first film, which gave us a villainous Leslie Nielsen, two years after Airplane! had made him a comedy icon?

Still, it would be equally remiss to not also note Romero’s declining years. Like Dario Argento, who he in fact worked with to produce Dawn of the Dead, George Romero’s output and quality inarguably declined in his twilight years, as he returned to the zombie concept in an apparent effort to find a sort of relevancy to contribute to the conversation. It’s not difficult to imagine that he saw the proliferation of low-budget zombie features as something of a challenge—something he tried to address in both Diary of the Dead and Survival of the Dead, without much success in the eyes of either critics or fans. History has seemingly judged these last two features as ultimately unnecessary—disposable films that in no way tarnish the legacy of one of horror’s greatest luminaries.

In the end, the legacy of George A. Romero is borne out in the legion of creatives that he inspired. The ‘80s zombie movie boom in the U.S. was the harvest of nightmares that were sown in the minds of young filmmakers who caught Night of the Living Dead at midnight showings across the country. The Italian zombie films of Lucio Fulci could not have been possible without Romero to show the way. Even the modern zom-com, from Zombieland to the Romero-named Shaun of the Dead, follow directly in his footsteps. There’s no other type of “monster” so indelibly associated with one, singular man. That’s the legacy he leaves behind.

We wish George A. Romero a much deeper, more peaceful eternity than that of his most famous creations.

Jim Vorel is a Paste staff writer and resident horror geek. You can follow him on Twitter.