|The following information is from the Adventure Time comic book series. It may not be part of the same canon or continuity as the animated series.|
Adventure Time #59 is the fifty-ninth issue of the Adventure Time comic book.
Finn finds his current adventure to be a bit more than he expected, but he's getting really into it. It's kind of like how someone approaches taking over their basement with a model train set.
The comic starts with a flashback to when Finn and Jake found the Blade of the Defender on the moon, but there is a mysterious person in a spacesuit watching them. The flashback moves forward to when they defeated Arklothac and told Janice Bizzlethatcher to never try something again. The possessor monster reveals itself from the sword, scaring Finn into throwing it in the ocean. The sword hits the ocean floor, and the monster leaves to possess a sunken boat. Afterward, the space person claims the sword and takes it through the portal to the moon, placing it back in the dungeon where it was found.
In the present, Janice is in the Temple of Arklothac planning to summon him properly this time. Meanwhile, Finn and Jake have brought one of the members of the Dungeon Masters Guild to check out the cave. He scoffs at them for not being able to feel the magic. Jake says that making fun of the inexperienced achieves nothing, and Finn adds that they aren't even wizards. The wizard apologizes and commends them on sneaking into Wizard City. Jake asks about the dangerous magic the guild was worried about. The wizard says that it's a force older than life and death, but he's curious about it. Jake tells Finn that Ng'zot Aa, who is still inside a computer, said something similar. Worried the wizard could be power-mad, they decide it better if they handle it. They soon come to a moon-shaped ruins they didn't see before. Finn realizes that the moon symbols seem familiar and wonders what the text reads. When he touches it, his mind is hit by something. The wizard can't read the text, but thinks it is about advanced dungeon crafting. Finn says he understands all of it now.
In the Candy Kingdom, Princess Bubblegum asks if they have enough stuff, and Peppermint Butler replies that they don't. She figures they'll have to make do, as long as they have the youth pot. She worries about the candy people, but Peps tells her that they'll be okay. At the temple, Janice manages to successfully summon Arklothac. In the cave, Finn states that it was his purpose to come and study dungeon making, but Jake isn't really convinced. Finn points out an image of him, but Jake thinks it's a bad picture of a bear. Then Finn points out an image of the Blade of the Defender, which Jake believes. Finn says that the sword is for someone who can "withstand great bodily loss without taking injury." It didn't work for him when he got hurt, and he wonders who it is for. At the tree fort, BMO feels something bad is happening. They are scared away by Arklothac, who is looking for Ng'zot Aa. He finds him in the video game he was captured in and pulls him out. Ng'zot Aa is glad to see an old friend again, but is feeling hungry. Arklothac says he will make a feast for them, but that the last of them should join first. He wakes up Zon, who turns out to be the sun, and calls him to join. Reunited, they embody the elements of slime, candy, and fire. They call the one who embodies ice, the moon, the betrayer due to caring about people. The moon was also keeping them asleep and failed. Janice realizes her mistake in judgment.
Back in the cave, everyone hears a rumbling sound. Jake thinks it might be a dungeon monster, but when they look outside, they see Arklothac, Ng'zot Aa, and Zon cooking and destroying the land. Bubblegum and Peps are fleeing from the destruction with their stuff, and Bubblegum wishes luck to her people. Finn thinks they can beat the three beings since they beat two of them before. The wizard guesses he can get help from other wizards, but also says he might just hide in another dimension. Finn says they need to get the Blade of the Defender from the water, but the space person appears and tells him not to, as it is back on the moon for the right warrior. Finn asks the space person who they are, and they say they are someone who knows what needs to be done, telling them to follow back into the cave. As they go, Finn thinks they should shoot him from the moon to stop Arklothac like the last time. The space person says that wasn't a permanent solution, and with the others also awake, they all need to be defeated together. They return the ruin, and the space person tells Finn that he has a role is saving Ooo. Finn and Jake question if the space person is the one who kept looting their dungeon, and the space person says no. They also say that it isn't a dungeon but part of the remains of Lun, the fourth of the race with the unpronounceable name and the only one to care about lesser beings. It sacrificed itself to be the weapons to destroy the others. The space person tells Finn to get into a pool of goo, which Finn recognizes as the same goo that shows different points in time and alternate realities. The space person explains that, like the race with the unpronounceable name (Jake wonders how they can say that), the goo is older than Ooo and is linked to time, space, and existence. Finn is skeptical because he doesn't know the space person, but instantly becomes trustful after they reveal their face to him. Jake is unable to go, as he needs to stay and help the princesses. He asks Finn if he'll be okay. Finn says he will and jumps into the goo. Jake then wants to see the space person's face, but finds that they are gone.
Finn emerges from a portal and ends up in the space station on the moon. He hears to people saying the project they're part of is shutting down and they'll be going back to Earth. Finn is confused at first, but realizes that he went to the past. He puts on a spacesuit and goes outside to get the Blade of the Defender, which he pulls out of a crater. Finn knows the sword needs a dungeon to protect it, and he now has the knowledge and time to make one. In the present, the three beings continue to ravage Ooo. The space person watches on, saying they protected the sword for so long and wondering how long it will be until the right person wields it. The space person is revealed to be an elderly Finn.