The Grass Sword was Finn's sword from the episode "Blade of Grass" which replaced Joshua's broken demon blood sword. He buys it from the Grassy Wizard, who cursed the sword so it will bond itself to its owner for eternity.
The sword is lost along with Finn's arm in "Escape from the Citadel" while attempting to hold on to a vessel carrying Martin, his dad, leaving only a flower embedded where it was. In "Breezy" Finn's arm regenerates, leaving only a thorn on his palm as the last trace of the sword. Since then, the grass sword regenerates only when Finn is in life-threatening crisis. Later in "The Comet", Finn has a new grass sword.
As of "Reboot", the Grass Sword detached itself from Finn and fused with the Finn Sword to create Fern. Fern can summon the Grass Sword at will, like Finn originally could, until his disintegration in the series finale "Come Along With Me".
The Grass Sword first appears in the episode "Blade of Grass." When Finn struggles to fight off monsters with the broken Demon Blood Sword, Jake tells him to get a new sword. They go shopping in town, where Jake leads Finn into the Grassy Wizard's teepee as he had heard it was "the place to get swords." Finn buys the grass sword for three dollars. As he leaves the teepee, the Grassy Wizard quickly whispers, "Remember, no take-backs."
Tree Trunks is the first to suspect that the grass sword is cursed. When Finn slices an apple for her, she observes, "Something is terribly wrong with that sword... That apple slicing is too perfect. Demonically perfect. That sword is cursed!"
Later that night, Finn stows the sword in a box before going to sleep. He dreams that he is standing in a grassy field; a sword appears out of the grass, dissolves into grass, and reappears in his hand. But soon, the grass spreads to the rest of Finn's body, and he too dissolves in the wind. He awakens to find that the sword has somehow made its way into his sleeping bag, and is now wrapping itself around Finn's right arm. Finn runs into the forest, tears off the sword, and kicks it into a river. But when he goes to the bathroom the next morning, he finds the sword inside, dripping wet. Jake tries to stop the sword from latching onto Finn by wrapping his apron around its hilt and sheathing it with a baguette.
Finn and Jake try to return the sword the next day but discover that the Grassy Wizard's teepee has vanished. With the help of Choose Goose, they track down the Grassy Wizard to his shack, where he unsuccessfully attempts to fend them off. Finn demands that he lift the curse, but the Grassy Wizard replies, "Never! You have no power over me, young fool! The grass sword will be part of you for all eternity!" After considering the sword's effectiveness, Finn realizes, "You know what? I think I'm cool with that." His acceptance of the curse allows him to control the sword's powers.
Throughout "Blade of Grass," Finn is able to use the sword with extreme precision. This is to the point that he can quickly carve his face into various objects. Also, when Finn flicks the sword, it whirls in deadly circles like a helicopter blade.
The grass sword also possesses abilities resulting from its curse. The sword incessantly follows Finn; when Finn tries to cast it away, he soon finds himself holding the sword again. When Finn accepts that the sword is cursed, he becomes able to control its material state—he can toggle between wearing the sword around his right arm as a bracelet and extending it as a sword.
The blade can also regenerate itself when cut off. The grass sword is able to turn parts of Finn's body into grass when he feels a strong need to accomplish something. In the episode "Escape from the Citadel," Finn's arm formed into a large, grassy, spiny arm in an attempt to keep his dad from escaping, only for it to come off taking Finn's arm with it, leaving only a flower growing in its place after being exposed to blood of the Citadel Guardians.
As of "Rattleballs," once Finn takes control of the grass sword, he has trouble controlling its extreme precision. In "Billy's Bucket List," the sword cuts the Fear Feaster in half of its own accord. The Fear Feaster also says that "no mortal blade can harm" him.
In "Escape from the Citadel," when Finn tries to prevent Martin from escaping, the sword entangles around Finn's whole arm and expands, and then snaps, taking Finn's forearm along with it. When the Citadel Guardian's blood got on the stub where Finn's arm once was, his arm didn't grow back, but it instead left behind a flower, which was likely a remnant of the sword.
In "Breezy" it's revealed that the condition of the flower is based on Finn's emotional state. In an attempt to rejuvenate the flower, Finn goes out and tries to make out with a lot of princesses. When a bee named Breezy falls in love with the flower on Finn's arm, she becomes his wing-man to help the flower stay alive. Over the course of the two hanging out, Breezy falls in love with Finn and sacrifices her independence to help him and the flower when he gets in danger. When she confesses her feelings, Finn politely rejects her, and later that night Breezy visits him in his sleep. When the two sing a song together, the Flower on Finn's arm rapidly grows into a tree branch and explodes. Underneath the bark of the tree is a regenerated arm. After Finn celebrates, he notices Breezy and witnesses the original flower floating onto her head. Finn new arm has a thorn on the palm, which also appears in the episode "Is That You?".
In "The Comet" Finn's regenerated arm is revealed to be the remnants of the grass sword turned into an arm. When Finn is attacked by Orgalorg, the arm's thorn activates and turns into a large vine-like appendage which Finn uses to free himself and the power of the Catalyst Comet from the inside of Orgalorg's stomach. After using it to free the Catalyst Comet, the tentacle is retracted, although Finn's arm is still covered in grass. However, his arm is normal again when he climbs out of Banana Man's rocket.
In "Checkmate", it is revealed that the sword is not entirely controlled by Finn when he tries to use it against the Vampire King. Finn tries to activate it when the Vampire King is close to him but is unsuccessful as the sword doesn't end up activating at all.
In "Crossover" the vine version of the sword appears again when Finn is frozen by Ice Finn. When Farmworld's version of the Lich tries to kill Jake, the sword arm activates on its own and attacks both the Lich and Ice Finn, cutting off the former's hand that was holding Jake and knocking the latter down onto the ground. The sword arm also freed Finn from the ice and calmed down after Finn told it not to kill the other Finn.
In "Flute Spell" Huntress Wizard takes note of Finn due to his unique flute improvisation. She claims that his arm is guided by a powerful spirit, and demands to know what it is. Finn reveals this to be his "grass arm" as he calls it, and says that when he plays he just lets the arm do what it wants.
In "I Am a Sword" it is shown that Finn can summon the whole sword separately from his actual arm. He uses the original Grass Sword for the first time in season 7 when fighting Bandit Princess, who had stolen and was using Finn Sword to fight him. While trying to land a blow on the thief, Finn accidentally runs Finn Sword through the gem with Grass Sword, breaking the gem and tearing the Grass Sword in half. The half that was run through and broken off in the Finn Sword is absorbed by the sword's gem. After BMO places a sticker of Finn's face onto the gem and Jake places Finn Sword above the fireplace, the combination of the two swords causes the gem to glow a bright green color.
In "Reboot," the Grass Sword actives after Finn is punched by an out-of-control Susan, despite Finn previously telling the sword not to attack her. After Finn punches his arm and forces the grass tentacles to retreat, they touch the Finn Sword. This results in the grass arm detaching from Finn and merging with the Finn Sword, which then turns into a strange grass humanoid (who later becomes known as Fern. In later appearances, Fern is able to summon the grass sword the same way Finn was able to). In "Whispers", he summons two swords, one in each hand.
In "Two Swords" it is revealed that within the Grass Sword is a sentient Grass Demon who interacts with small Finn in the Finn Sword upon breaking it. The Grass Demon and small Finn together are within Fern controlling him.
Fern is accidentally killed by Finn in "Three Buckets". Fern's remains are just grass (and his eyeball) However, he, and by extension, the Grass Sword, are revived by Uncle Gumbald, who allies with him. In "Seventeen", Fern is genetically modified to become the Green Knight and tries to kill Finn, but is stopped by Marceline.
In the series finale "Come Along With Me" while in hallucinatory slumber, Finn and Fern removed the Grass Demon from Fern (or simply Finn), however without the Grass Demon, Fern can have no body. Fern began to disintegrate, which he eventually did, leaving a seedling in the shape of the Finn Sword. The seedling was planted by Finn and Jake at what remained of their destroyed Tree Fort. The seedling immediately grew into a full tree similar to the tree fort with a new Finn Sword growing out of it. The tree continued to grow, becoming massive 1000 years later, with Shermy and Beth taking the sword out. It is unknown if the Grass Sword was destroyed completely or if it was what formed the seedling and is apart of the tree.
- "Blade of Grass"
- "The Red Throne"
- "Bad Timing"
- "Lemonhope Part 1"
- "Lemonhope Part 2"
- "Billy's Bucket List"
- "Wake Up"
- "Escape from the Citadel"
- "The Comet"
- "I Am a Sword"
- "Two Swords"
- "Do No Harm"
- "The Invitation"
- "The Light Cloud" (flashback)
- "Three Buckets"
- "Come Along With Me"
- This sword only existed for 10 episodes, making it the shortest time that Finn owned a certain sword. However, it still technically existed in every episode with Finn after it was seemingly removed from his body, in the form of his flower from "James II" to "Breezy," and his new arm from "Breezy" to "Reboot." Fern was also made of it and summoned it the way Finn did between his debut in "Reboot" and his death in "Come Along With Me", it may even exist within the Fern sampling and tree. Factoring this in, it can also be the longest time Finn owned a certain sword.
- The Grass Sword and Finn Sword merged together, creating Fern. Incidentally, the Finn Sword had an alternate Grass Sword within it, as the alternate Finn had the grass sword in his right arm when he became the sword.
- In Adventure Time Battle Party, you can see Finn wielding the grass sword in his default costume.
- Finn carries the grass sword on his right arm, which is inevitably lost by his alternate forms: Pillow World Finn, Farmworld Finn, and Shoko. Eventually, Finn does lose his right arm and the sword with it, though a part of the sword eventually regrows into his new arm.
- The grass sword is one of the many swords available to Finn in LEGO Dimensions. It has the special abilities to cut vines and can also be used as a rope and grappling hook.
- Cursed weaponry that cannot be unequipped is a common video game theme; however, the most likely origin of cursed items is Dungeons & Dragons.
- Another explanation for the grass sword's behavior is that it is imbued with some sort of intelligence; it returned to Finn after he tried to get rid of it. In traditional tabletop RPGs a cursed item usually cannot be removed without the aid of a "remove curse" spell, and since Finn was able to let go of the sword after using it for a day, it is evident that the grass sword is an intelligent item that may or may not be cursed. Furthermore, in "Billy's Bucket List" the grass sword slays the Fear Feaster seemingly of its own accord.
- The grass sword might be a reference to the "Blade of Grass" from the game Terraria.
- It is similar to the Pokémon move Leaf Blade, which consists of slashing at the opponents with a glowing grass blade from the arms.
- The Grass Sword is very similar to Keldric's weapon from Dawn of Mana, as both weapons have plant-like designs and are wrapped around the both character's right arm when not in use.
- The grass sword has a similar design to the kampilan, a Philippine sword.
- The most likely inspiration, however, is the kusinagi-no-tsurugi or grass cutting sword which is part of the imperial regalia of Japan where the sun goddess Amaterasu passed the sword down to her great-grandson who would become the first shogun of Japan.
- The grass sword's connection to Finn's arm is similar to Ashitaka's cursed arm in Studio Ghibli's Princess Mononoke; both have minds of their own and give the possessor superhuman strength. Finn and Ashitaka also share many traits, such as being noble heroes and have animal "traveling companions."