It Came from the Nightosphere

"It Came from the Nightosphere" is the first episode of Adventure Time's second season.

Plot
The episode begins with Marceline and Finn hanging out at her evil but tastefully appointed lair. She asks Finn to help her lay down a sweet track with her, but to remain non-judgemental as to the extremely personal nature of the song. Finn agrees, but upon hearing the song, a mournful tale of parental estrangement and french fry theft, Finn asks Marceline why she doesn't reconcile with her Father, as the song suggests is her desire. Marcelene explains that not only do they have longstanding difficulties in the daddy/daughter department, but also that he is trapped incorporeally in a nighmarinsh pocket-dimension known as the Nightosphere. The only way to free him is to draw a circle of summoning engraved with his sigil, douse it with bug milk, and chant a fell incantation in Latin. Finn immidiately does so, and reality shudders as the veil between our world and the Nightosphere is torn asunder, granting freedom to the Lord of Evil, Sunderer of Worlds and Extremely Snappy Dresser. After dissuading him from devouring Finn's soul on the spot, he takes Marceline's bass, actually a family heirloom axe which she converted to shred riffs instead of flesh, and stalks off into the night to suck the souls from the hapless denizens of Ooo. Finn rushes to stop him, Marceline in tow to retrieve her beloved guitar. Marceline's father embarks on a cross-country feast of souls, tormenting and draining the life from Fluffy People, Marauders and even the Penguins of the Ice Kingdom, where Gunter impresses the Dark Lord with his evil chops. The Ice King confronts the Unfathomable Dweller Between Worlds, but he shows the poor magic-user a glimpse of his true form, a squamous eldrich horror which would shatter the sanity of lesser men, but which merely causes the Ice King to poop his robe. He tries to recover from his embarrassment by saying "he did it intentionally" as he flies away. Marcilene and Finn set an ambush in a mountain pass, but an enraged Marceline charges her father to take back her bass. She is soundly defeated, and he continues his grim harvest, consuming even such notable beings as Hot Dog Princess and Lumpy Space Princess, who dove willingly into the mouth of madness because she assumed there was a party within, rather than slow ghastly spiritual disintegration. His additional meals begin to allow him to manifest aspects of his true form, a brain-meltingly hideous creature of alien geometries and pulpy tentacles. Finn, after awkwardly making double entendres while trying to come up with a metaphor for fighting him, charges headlong at the slavering abomination in an attempt to leverage his favorite problem solving tool, violence. He is swatted like an insect. Marceline retrieves her bass and storms off, flustered. Desperate for a distraction, Finn plays the recording they had made earlier of Marceline singing about her father, causing her to blush furiously. Marceline's father assures her that he does indeed love her, and he is sorry for stealing her fries, and they have a tender moment of reconciliation. Finn takes this opportunity to gut him like a pinata made of rotting fish, spilling his entrails on the ground and freeing the trapped souls. Though immortal and exceptionally powerful, The Lord of Evil is surprised momentarily, and Finn is able to banish him to the Nightosphere before he regenerates. The two collapse in an exhausted heap, and the episode ends as we recieve insight into the contents of Finn's breast pocket, that being a particularly flatulent Jake the Dog. 

Characters

 * Finn
 * Jake
 * Marceline
 * Marceline's Father
 * Tree People
 * Fuzzy Friends
 * Penguins (including Gunter).
 * Marauders
 * Rock People
 * Lumpy Space Princess
 * Ice King

Trivia

 * This episode previously was titled "Return to Nightosphere" and just "Nightosphere".
 * This is Pen's favorite episode of the second season.
 * According to the Frederator Blog, this episode was not completely finished one week before it aired.
 * This is the first episode storyboarded by Rebecca Sugar, and to commemorate that fact, she made a special drawing.
 * This is the first episode where Jake only receives a minor cameo and has no lines.
 * The smiley face used to open the Nightosphere is a Phil face.