Nightosphere

The Nightosphere is a space or dimension that Marceline locked her father in. Portals to this dimension can be opened by drawing a smiley face in a circle, dousing it in bug milk, and reciting an incantation, as Finn and Marceline did in "It Came from the Nightosphere". The only clue to the appearance of the Nightosphere is that the portal to it is full of fire.

Trivia

 * When drawing the smiley face, Finn drew a Phil face.