Grass Sword

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. It returns however, in "The Comet," where it was revealed to be Finn's new arm he got in "Breezy" under a layer of skin.

Abilities
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.

As of "Rattleballs," once Finn takes control of the grass sword, he has trouble controlling its extreme precision. 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 exposing to blood of the Citadel Guardians.

In "The Comet", Finn's arm is transformed into a large, grassy tentacle which he uses to cut the tentacles draining the power of the Catalyst Comet inside Orgalorg's stomach, despite the arm the sword had previously been attached to having been ripped off.

Origin
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."

Curse
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.

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.

At the end of the episode Breezy, after a bee grants Finn's right arm back, there is a stub of what looks like a grass root on his right hand, hinting that the curse has not been removed.

In "The Comet", Finn's arm transforms into a grassy tentacle which confirms that the curse is not gone. 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.

Finn tries to use this ability again in "Checkmate" against the "Vampire King", but fails to.

It was not until "Crossover" when he was able to use the Grass arm again in saving Farmworld Finn from the Lich.

In "I Am a Sword", the Grass Sword breaks again, while Finn uses it to fight against Bandit Princess, resulting in Finn accidentally stabbing the Finn Sword.

Episode appearances

 * "Blade of Grass"
 * "Rattleballs"
 * "The Red Throne"
 * "Betty"
 * "Bad Timing"
 * "Lemonhope Part 1"
 * "Lemonhope Part 2"
 * "Billy's Bucket List"
 * "Wake Up"
 * "Escape from the Citadel"
 * "The Comet"
 * "Crossover"
 * "I Am a Sword"

Trivia

 * 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" onward.
 * 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.

Cultural references

 * 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, 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.

Screenshots
Espada de Pasto