Nightosphere

The Nightosphere is a place that someone locked Marceline's father in. Portals to this dimension can be opened by drawing a PHIL FACE, 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. When Marceline's father is set free in that episode, Finn ultimately sends him back to the Nightosphere at the end.

How to get there

 * 1) First draw a PHIL FACE.
 * 2) Then douse it with Bug Milk.
 * 3) Then chant “Maloso vobiscum et cum spiritum.”

Trivia

 * The smiley face Finn drew was a PHIL FACE.
 * As said by Adam Muto, it is impossible to send The Lich to Nightosphere because he's not from there. There are some unknown rules for the banishment.
 * "Maloso vobiscum et cum spiritum" roughly translates from Latin to "Evil be with you and your spirit."
 * Due to its amount of fire, it looks like it might be a sealed place in the Fire Kingdom.
 * Another possibility is as a sugestive answer of Adam Muto, the Nightosphere could be the Underworld, cited in Heat Signature.
 * The nightosphere is most likely hell, as Finn states "I'll see you in the Nightosphere..." as this appears to be a refrence to the common phrase "I'll see you in Hell."