Locations, Locations...

The Nightosphere

 http://adventuretime.wikia.com/wiki/Nightosphere_(location)

The Nightosphere in an alternate dimension in The land of Ooo. It is essentially  Adventure Time's equivalent to Hell. It's inhabited by Hunson Abadeer (who doubles as the ruler of the Nightosphere and Marceline's Dad) and an enormity of other demonic entities. 

http://www.watchcartoononline.com/adventure-time-episode-14-it-came-from-the-nightosphere

The Nightosphere first appeared in the episode "It came from the Nightosphere", where a portal was formed in Marceline's house and led to a land engulfed in fire. The location was later established in "Return to the Nightosphere" and later in "Daddy's Little Monster", where it was portrayed as a demonic wasteland immersed in constant chaos. The only way to escape the Nightosphere is by going through a portal in Hunson's house.

http://www.watchcartoononline.com/adventure-time-episode-14-it-came-from-the-nightosphere

How to Get To The Nightosphere

1. Draw a happy face (it must be a Phil Face) on any surface

2. Drench the face in bug milk

3. Chant, "Maloso vobiscum et cum spiritum!"

4. Enter portal

 http://adventuretime.wikia.com/wiki/Nightosphere_(location)

                            The Phil face and bug milk

Factz

• The chant you have to perform to enter the Nightosphere, "Maloso vobiscum et cum spiritum", translates from Latin to"Evil be with you and with your spirit." This could be considered a reference to Catholic liturgy, where a priest says, "Dominus vobiscum" ("The Lord be with you"), and the congregation replies, "et cum spiritu tuo" ("and with your spirit)
• In one of the episodes where Finn is escaping the Nightosphere, he shouts out, "I'll  see you in the Nightosphere, you sick freak!", which bares resemblence to the phrase, "I'll see you in Hell"
• The demons in the Nightosphere poop bananas...out of their ears

 http://adventuretime.wikia.com/wiki/Nightosphere_(location)