Cartoon Network premiered one of its darkest, raunchiest series of all time, The Grim Adventures of Billy & Mandy, an animated show about two children enslaving and befriending the Grim Reaper, 15 years ago this week. Originally conceived as half of the show Grim & Evil, it told the tale of two best friends — the dimwitted yet enthusiastic Billy and the cynical, disturbingly intelligent Mandy — who force the Grim Reaper to be their best friend forever after defeating him in a game of Limbo.

Grim & Evil's initial existence was largely due to a viewer poll. It originated as part of Cartoon Network's Cartoon Cartoons (originally known as What a Cartoon!), a cartoon incubator, for which the network would air a series of shorts and assess their potential to become full spin-off series. The first spin-off was Dexter's Laboratory in 1996, after having aired its first short in February a year earlier, followed by Johnny Bravo and Cow and Chicken. In 2000, Cartoon Network did things a bit differently with its 53-hour marathon event called The Big Pick, in which viewers got to vote for which of 10 pilots would become a half-hour show for the fall season. The first show to win The Big Pick was Grim & Evil with 57 percent of the votes.

The Evil half of Grim & Evil was Evil Con Carne, the tale of a "jillionaire playboy" evil genius whose body blew up but whose brain and stomach were salvaged and placed in a purple Russian bear. The character was later salvaged in his entirety and would pop up as a villain from time to time in Billy & Mandy.

Unlike the evil bear, though, Billy and Mandy weren't part of the original series pitch. Creator Maxwell Atoms recalls that the unlikely best friends were secondary characters in another potential show.

"I originally pitched a superhero comedy show called Milkman, and Billy and Mandy were side characters," Atoms told SYFY WIRE. "The people at Cartoon Network picked those two characters and asked if I could make a show about them, which was shocking. Then somewhere in the middle of it, they decided they needed a third character. I was working on a bunch of ideas at the time and said, 'Well, let's put the Grim Reaper in this, why not.' Cow and Chicken had the devil, and we got the Grim Reaper."

And so it was that two little kids tricked the Grim Reaper into betting his life in a game of limbo to try and save Billy's hamster's eternal soul.

The Grim Adventures of Billy & Mandy would run for 86 episodes, including the original Grim & Evil shorts, as well as three movies and two special episodes. It also won two Emmy awards.

Atoms was always interested in cartoons from the time when you actually had to wake up on a Saturday morning or you would miss your favorite shows. The dark humor and raunchiness of Billy & Mandy was something he was always interested in, even if that was uncommon in children's cartoons at the time.

"I had been toying around with this idea for years. I used to do a comic when I was a kid called Billy in Space, about a different kid [from the Billy in Billy & Mandy] who kind of looked the same," Atoms says. "Then in college, I was into all sorts of weird fringe culture stuff, and there was this surgery called 'Trepanning' where you just bang a hole in your head to let the evil out. So my junior year of college I made a two-minute short film about these two little kids putting holes in each other's head."

One of the things that made Billy & Mandy unique for the era was how dark it could go at times, with constant references to horror and the occult, and many an episode focusing on horror tropes while paying them for laughs. An entire episode is dedicated to a parody of Stephen King's Christine and features a killer tricycle that terrorizes the neighborhood. In spite of the show's tone, Atoms recalls Carton Network never really opposing the weirdness, just other things.

"The shocking thing is, everything we did was approved by Cartoon Network. You can't really sneak things past them. But it was extremely rare for them to flex their muscle. We just did every weird and raunchy thing we could think of," Atoms says. "And you know, every show has a Standards Department and they are different in every network, so it all comes down to negotiating what you want versus what they need.

"So you would get into these silly conversations where you would go 'Well, last week in Phineas and Ferb they showed somebody's butt, so we should be able to show somebody's butt.' And they would go, 'Let me go watch that episode… OK, you can show a butt.' The weird thing is, the scary stuff or the monsters and tentacles, maybe because there wasn't a lot of [that] in cartoons at the time, they didn't know what to do with it. Much of the time they would just ignore that and just go 'Hey, is this a real fart noise here?' which made it easier for us."

Video of Dracula Don't Suck, Dracula Scrape and Lick

In addition to its raunchy, dark sense of humor, the show was filled with pop culture references, all twisted to fit the tone of the Billy & Mandy universe. Where else could you see Yogi Bear and Boo Boo depicted as junkies with bloodshot eyes, scratching their arms in torn clothes? Even Blacula became a recurrent character.

"We did a lot of mash-ups like that, where you take something that's subversive, but it also works on a comedy level," Atoms says. "Like a Billy Elliot episode that also follows the plot of Suspiria. I think I was always more aware of trying to steer it more toward kids, while a lot of the animators would just put in whatever cultural references they thought were funny and it ended up being somewhere in between. One of my goals was to scare kids right before we make them laugh."

One of the joys of watching the show as an adult is picking up all the references, from the horror movies as mentioned above, to the most obscure things you would never think a Cartoon Network show would reference. In the third season episode "Mandy the Merciless," Billy, Mandy, and Grim decide to look into the future for fun. They see a world ruled by Mandy as an immortal half-worm, half-human demon dictator who controls the world by hoarding cinnamon and ultimately learns of a rebellion through precognition. As a kid, it is not easy to spot the many references to Frank Herbert's God Emperor of Dune novel, and that's why Atoms loves that episode.

"That is such a weird and obscure reference — it's probably my favorite parody we ever did," he says. "This was a very post-modern show and I don't know if I even could do another one like that, let alone have it greenlit today."

Among the many minor characters in The Grim Adventures of Billy & Mandy, one ridiculous character in particular, Fre Fredburger, got so little explanation yet ended up being beloved by the fans. In the Season 6 episode "Keeper of the Reaper," Billy's family is moving out of town, so he takes Mandy to court to fight over custody of Grim. In the jury, we meet a little green elephant monster named Fred Fredburger.

Video of Fred Fredburger Moments

"He is almost completely the creation of Carl 'C.H.' Greenblatt, who created Chowder and Harvey Beaks at Nickelodeon," Atoms says. "He was one of our board artists in Billy & Mandy and our board artist basically wrote the show as well as drew it. 'Keeper of the Reaper' was an episode we worked on for a while and it wasn't coming together as we expected, and one day he comes into my office and says, 'Hey, I have an idea. We're going to make the whole courtroom thing, and we're going to base it around this guy. Who is just like a regular guy, and he's like a little elephant.' And we knew the second he pitched it that Fred had to come back later."

Atoms is now working on a new animated show but admits it's taking him longer than expected because of the lower budget. When it comes to The Grim Adventures of Billy & Mandy, he, to this day, considers himself lucky.

"They gave me so much freedom to tell the story that I wanted to tell," Atoms says. "And I also had a really fantastic crew to back it up. I've worked a couple of other jobs since then, and it all really makes me appreciate what I had."