Adventure Time’s Grass Sword changed the show in ways nobody expected

That platonic ideal weapon, known by fans as the "Adventure Time Sword" never actually appears in the show itself, but there's no shortage of memorable blades wielded by Finn and various other characters across 10 seasons and various spinoffs. There's Finn's original sword, which defines the show's early seasons with its scratched and dented blade. There's the iconic Root Sword with its naturalistic wooden hilt. And there's the mind-bending Finn Sword, created out of a time-travel paradox after Finn runs into an alternate-reality version of himself and the copy instantly transforms into a weapon.

But the most fascinating (and in this longtime fan's opinion, coolest) sword in Adventure Time history is without question the Grass Sword, which begins as a cursed object with a mind of its own before taking a sharp left turn and eventually transforming into Fern, a grassy Finn doppelgänger who became an antagonist and ultimately an ally. Like so much in Adventure Time and its sprawling, cosmic mythology, that was never the plan. The story emerged naturally as the show's creators followed their instincts, one episode at a time.

"When we came up with the Grass Sword, I don't think there was ever the notion of like, Oh, this will lead to a grass version of Finn," Adventure Time showrunner Adam Muto tells Polygon. But the Grass Sword took root in the collective imagination and blossomed across seasons.

To celebrate Cool Sword Day, Polygon caught up with Muto to unpack the origins of the Grass Sword and its evolution into Fern.

The Grass Sword

"We would go through swords pretty quickly," Muto says.

While for the first few seasons of Adventure Time, Finn relied on his trusty, beat-up blade (called The Golden Sword of Battle, or "Scarlet"), after that sword was lost in a series of events too complex to explain here, the team had free reign to experiment with various options.

"We'd periodically do these bake offs where it was like, we need a new sword," Muto says. "So then the designers would just submit a bunch and the storyboard-artist-writers would also do sketches."

By season 5, Finn was wielding the also incredibly cool Demon Blood Sword, which he found in a dungeon. After that sword breaks, Finn and Jake go shopping for a new one in the season 5 episode, "Blade of Grass."

Meanwhile, in the writers' room, Muto, series creator Pendleton Ward, and the rest of the team had been debating what Finn's next sword should be. The group had an idea that, down the line, Finn would lose an arm (more on that in a second), while at the same time, they were playing around with a variation on a classic concept.

"We just started spitballing a story about a cursed sword because that's a pretty traditional trope," Muto says.

It all came together when Ward had the idea for a sword of grass, another riff on a classic trope that Adventure Time could make its own.

"There've been lots of swords made out of grass with the blade of grass pun," Muto says. "So [Ward] felt comfortable using it. It didn't seem like it was owned by one specific fantasy world."

The design took a bit of fine tuning, however. The original version of the Grass Sword as imagined by Ward would have grown out of Finn's arm in a "very invasive way."

"I was like, 'That's too gross,'" Muto recalls.

Then, they landed on the solution, a blade of grass that could wrap itself around Finn's arm when it wasn't in use — kind of like a slap bracelet. Not only did this look cool, but it solved an ongoing problem for Adventure Time's artists: It finally gave them a place to put Finn's sword when it wasn't in use.

Finn never carries a scabbard, so when he wasn't holding whatever sword he had at the time, he'd typically just shove it between his back and his backpack. The Adventure Time team disliked this solution, but they were also opposed to using cartoon logic where the sword could just disappear into Finn's bag when it wasn't in use. The slap-bracelet design was an elegant solution.

"It was the one exception where this sword could basically come out of nowhere like magic and then go away," Muto says. "It was just a very cool visual."

In the episode, Finn buys the Grass Sword from a sketchy shopkeep, only to realize too late that the item is cursed. The episode delves into horror territory as the sword stalks Finn after he tries to get rid, but he ultimately realizes that, if he stops fighting the Grass Sword, it's actually pretty sweet.

"The joke was that Finn doesn't care because it's cool," Muto says. "He's like, eventually maybe I'll have to deal with it."

Finn loses his arm

At the start of season 6, the Adventure Time team finally made good on its plan to remove Finn's arm. Near the end of season 5, Finn learns that his long-absent father was actually still alive. Season 6's two-part premiere finally introduces his dad, Martin Mertens, who is being held in the Crystal Citadel, a prison located in an inaccessible dimension where celestial beings guard the worst criminals in the multiverse. So our heroes plan a jailbreak.

Finn's dad turns out to be kind of a jerk, although he's more of an amoral narcissist than an actual evil guy. During a moment of chaos in the episode, Martin breaks free and attempts to ditch his son and escape into another dimension. When Finn tries to stop him, the Grass Sword suddenly transforms into a massive, brolic extension of Finn's arm. Under intense physical pressure, his arm pops clean off, leaving behind a smooth stump.

"Because that was such a big change to the model, we had to have these endless discussions with the network," Muto says. "Can you do this? Is anybody going to object too much or will it be too insensitive? Or will some countries just not air it because you have this dismemberment happening? So it had to happen in a magical way and it had to have an outcome that left room for regrowth."

Both the Grass Sword and Finn survive, but their relationship would never be the same. Finn gets a robot arm courtesy of Princess Bubblegum, and later in season 6 he finds (creates? becomes?) the Finn Sword.

For Muto, abandoning the Grass Sword was a natural step for Adventure Time, but a difficult one nonetheless. However, there was one big benefit to moving on from Finn's cursed weapon.

"I was kind of bummed when we had to get rid of it," Muto says, "but it was also always there on his arm. So as a design element, we'd have to go through and double check like, OK, do we remember to draw the Grass Sword? Is it all the right arm?" It became this extra detail that we had to track, and I think there were probably some scenes where we forgot it."

Of course, it wasn’t the last time we saw the Grass Sword in Adventure Time.

The Grass Sword becomes Fern

In the aptly titled season 8 episode "I Am A Sword," the story of the Grass Sword took its darkest turn yet. Finn and Jake are tracking down the villainous Bandit Princess, and when they finally confront her, she manages to get her hands on the Finn Sword. The Grass Sword suddenly materializes from a thorn in Finn's palm and a fight ensues, but in a shocking turn, the Grass Sword pierces the blue crystal that's set into the hilt of the Finn Sword. The blade of grass then seems to wilt before slithering inside the crystal. At the end of the episode, Finn covers the hole up with a sticker and hangs his sword on a wall of the treehouse as a mysterious green light shines from within it.

"If we want to get rid of the Grass Sword, so we're not constantly dealing with it, the only place it felt like it could go to is another Finn," Muto explains. "It seemed like a very striking visual because Finn was really attached to the Finn Sword and now it's been infected by the Grass Sword. So what would that lead to?"

For the Adventure Time team, this kind of open-ended approach was exactly the way they liked to work.

"There's a lot of that where we put the characters in a situation where it was like, What would be a really crazy, interesting visual or what would be a weird idea to explore? And it just kind of kept going," Muto says.

A few episodes later, a mysterious green figure begins to stalk Finn. "Grass Guy” is soon revealed to be a tiny version of Finn who lived inside the Finn Sword crystal. When it was pierced by the Grass Sword, tiny Finn got a new roommate: a creepy green demon who slowly consumed him. Their fused identities became a grass-based version of Finn the Human who names himself Fern (voiced by Hayden Ezzy).

Fern's story is pretty tragic, fueled by a desire to replace Finn. But faced with both his inability to measure up to the original (and a non-enthusiastic response from everyone he meets, Fern eventually breaks bad.

"When we introduced Fern, the obvious trope is: It's an evil twin story so something is going to be wrong about them," Muto says. "So initially we tried to diffuse that by having him just be this really confused guy who basically thought of himself as Finn, and when he realized he wasn't, it kind of messed with his sense of identity. IIt was sort of a slow unraveling influenced by the grass demon presence that was still inside of him. If he can't be his own person, he might as well try to be Finn, but if he can't be a better Finn, he has to get rid of the other Finn."

Ultimately, things go poorly for Fern, and Finn winds up killing him in one of the series’ more traumatic moments. However, Fern wasn't actually gone and showed up again in Adventure Time's final season, first as villain called the Green Knight, and ultimately as his original "Grass Guy" self where he gets a hero's death. He also showed up more recently in the second season of the spinoff series Fionna and Cake where we learn he's ascended to a higher plane of existence (sort of, like everything in this show, it's complicated).

The future of Fern

Fern's story arc might seem pretty complete — from cursed sword to evil doppelgänger to tragic hero — but Muto still sees potential for the character.

"There was this span of time where he was presumably having his own journey of self-reflection," Muto says. "We just didn't see most of that on camera."

Could we ever get a Fern spinoff in the style of Fionna and Cake? Adventure Time's longserving steward doesn't rule it out. In fact, it's something the team has already considered.

"We pitched ideas around him because it's such a strange performance, too," Muto says. "The voice actor, Hayden Ezzy, brought his own sort of idiosyncrasy to it. So it didn't sound ever like he was doing a Finn impression. He just seemed like this unique character, and his design is really cool too. So it would be cool to go back and find a way to use him. But it's also cool that we just got a glimpse of what his life was like and he found some peace, even if he's moved on to a different level of consciousness."

Bonus: Adam Muto's favorite sword in Adventure Time

Before my conversion with Muto ends, I ask him one more question: What's his favorite sword from the show?

While Muto also loves the Grass Sword, the Finn Sword, and countless others, he shouts out a surprising choice, a blade only seen one-time on the show in an episode where Finn and Jake visit Jake's brother, Joshua, and find a sword that looks suspiciously like one from another beloved franchise: Cloud's Buster Sword in Final Fantasy 7.

"The artist, Jesse Moynihan, wasn't even aware that it looked like Cloud's sword, but he accidentally did the same design of it," Muto says. "That one also stands out, but only because of the weird synchronicity of it. I don't think Jesse played Final Fantasy, but he accidentally designed this oversized sword that was pretty close to Cloud's."