In "Two Swords" it is revealed that during "I Am a Sword" when Finn accidentally cracked the Finn Sword with the Grass Sword, fighting Bandit Princess the interior of the sword has become filled with grass. The grass demon then appeared spraying grass webs around the sword's interior. The Grass Demon described itself as "a curse, an emissary from beyond, some kind of grass octopus-spider." The Sword Finn then called the creature a grass demon. The grass demon wrapped the Sword Finn in a grass cocoon and the two both desired to get out. When they "got out" they merged into a being with both their memories of personalities, including both real Finn and sword Finn along with the grass demon.
In "Whispers", the Grass Demon has many grass webs surrounding the inside of the sword. The webs conform to the inside of the Finn Sword as little Finn is shown slowly becoming controlled by the Grass Demon. While the Grass Demon is controlling little Finn Fern thinks up a plan to become "the only Finn".
Fern is noticeably more violent and hostile than the real Finn, likely because of the Grass Demon, controlling him. By "Three Buckets" it is seen that the Grass Demon began to control Fern even more, becoming nearly evil, attempting to kill Finn and showing the same demonic eyes as the grass demon. Fern was nearly killed by Finn, but his remains were retrieved by Gumbald and he transformed him into the Green Knight, and he continued to be evil (via Grass Demon) wanting to kill Finn.
In the series finale "Come Along With Me" Finn and Jake, wanting to stop the Gum War, unleashed the nightmare juice given to them by Nightmare Princess, forcing them, Fern, Princess Bubblegum, and Gumbald into a hallucinatory slumber. In the dream, Fern transformed into a bird/pterodactyl-like and flew away, while Finn decided to follow him after turning into a butterfly. Finn tried to calm Fern down, and two plummeted to the ground. Finn attempted to appeal to Fern by reminding him that they are "two sides of the same coin" and that they share the same torment. However, Finn and Fern continued to fight, but were able to find common ground after Jake brought them Finn's vault. Inside the vault, the Grass Demon appeared wrapped around the Sword Finn. Together Finn and Fern went in to confront the Grass Demon. However, the Grass Demon said that without him, Fern has no body and then tells Finn to back off. Despite this, Fern killed it, ridding himself of its influence once and for all. Afterwards, the Cosmic Owl flew by, making the dream come true.
Sitting on a beach, Princess Bubblegum touched Fern and her hands went right through him as he had no body. The group then awakened and Fern began to disintegrate as he could have no body.
The demon is a small creature, roughly the size of a human head, that resembles its personal description as "some kind of grass octopus-spider". It has five legs, and a tiny hole on top of its head that can shoot the grass webs.
- Web shooting.
From its initial appearance, the Grass Demon maintains a calm and a somewhat cheerful disposition. It seems to enjoy covering surfaces with a sticky substance similar to spider silk as it coated "interior" of the Finn Sword with its webs.
Is waiting for the rising...
Without me, he's got no body. So back off!
—Grass Demon (his last words before got killed)
- "Two Swords"
- "Three Buckets" (glowing eyes)
- ”Come Along With Me” (Nightmare Juice)