Nightosphere

The Nightosphere is an alternate dimension appearing in "It Came from the Nightosphere" inhabited by Hunson Abadeer and a multitude of other demonic and "deathless" entities. Though initially the only clue as to the appearance of the Nightosphere is that the portal between it and Ooo formed in Marceline's House led to a place engulfed in fire, details of the place were eventually exemplified in Return to the Nightosphere and Daddy's Little Monster, where it was viewed to be a demonic wasteland filled with chaos and many unhappy and unsatisfied demons.

How to get there

 * 1) First draw the PHIL FACE (happy face) in the center.
 * 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 would be impossible to send The Lich to the Nightosphere because he's not from that universe. There are some unknown rules regarding banishment.
 * "Maloso vobiscum et cum spiritum" roughly translates from Latin to: "Evil be with you and with your spirit." This phrase may be a reference to the Latin phrase translated to: "God be with you and with your spirit"
 * Due to its fiery atmosphere the Nightosphere appears similar to the Fire Kingdom.
 * A theoretical answer by Adam Muto is that the Nightosphere could be the Underworld cited in "Heat Signature."
 * The Nightosphere bears similarities to Hell in appearance, and Finn states, "I'll see you in the Nightosphere you sick freak." This is similar to the known phrase, "I'll see you in Hell."
 * According to Adam Muto, the Nightosphere has a demon jail only to maintain the illusion of order.