Devildom

The Devildom is one of the three worlds that exist in the game. It is where demons reside, and where the majority of the game takes place. The Devildom is a distinctly different realm from Hell, presumably existing in a plane just above it.

Overview
There are eight layers to the Devildom, and as mentioned during the normal story mode, The Demon King — Lord Diavolo's father — lies willingly dormant at the very bottom. Due to this, Lord Diavolo rules as the de facto king of the Devildom.

In the Devildom, there is no sunlight. The world exists in a constant state of darkness at all times, though according to Diavolo there are different degrees of darkness depending on the time. Despite this, demons still adhere to a "day/night" schedule where they sleep and are expected to awaken in the "morning" as evidenced by the fact that the demon brothers often sit down together for breakfast and dinner, as well as go to sleep and attend school/work at specific times.

The sky and its constellations are different from the human world's. In the Star-Crossed Lovers Devilgram, an autumn sky constellation is mentioned — the Pool of the Abyss — along with the myth associated with it. One star represents a beast, and another the maiden that was carried off by it. The nebula below the two stars symbolizes the pool created by her tears as she yearned to see her beloved.

In lesson 47-9, Belphegor mentions "Devildom script", meaning the Devildom likely has their own language.

It is stated in Season 3 that the Devildom once went to war with the Celestial Realm. It is unclear what the outcome of this war was, but this war ended long before the Great Celestial War.

Creation
In Lesson 30-1, Satan and Simeon expand on Devildom history. The Devildom came into existence alongside the Celestial Realm and exists in tandem with it. Shortly after its creation, a primeval forest covered the entirety of the Devildom.

Rulers
The Devildom is currently ruled by Lord Diavolo, but was previously ruled by his father, the Demon King. Diavolo stated in Lesson 7-10 that he had not seen his father in several hundred years, as he'd lost interest in ruling and was slumbering in the lowermost layer of the Devildom. Diavolo's father was still awake when the brothers fell, as he assigned them their titles as the avatars of sin.

Lucifer spoke in Lesson 7-10 of another Demon King who ruled prior to Diavolo's father. Nothing is currently known about this prior king, save that his crown is displayed in the Demon Lord's Castle.

Halloween
Halloween is considered a very important holiday in the Devildom, as it also being a celebration of Diavolo's birthday. Celebrations of this event last for three days.

Thanksgiving
In the Pop Quiz You're A Gem, it is shown that Thanksgiving in the Devildom involves the raining down of magical gemstones from the sky. Handling these gems causes both demons and angels to enter an extremely positive mood and express what they are thankful for. Giving these gems as gifts to loved ones is customary, and each colored gemstone has its own unique meaning. As Diavolo explained to MC, a special kind of golden gemstone only rains down around high-ranking demons.

New Years
In the Pop Quiz A Special Ceremony, Lucifer mentioned that New Years celebrations happen every 100 years in the Devildom.

Jill's Day
This holiday is mentioned in the Devilgram The Rare Flower. According to Lucifer, it is named after a woman who sold her soul to a demon. To celebrate, a tea party is hosted at the Demon Lord's Castle and Diavolo is presented with extravagant gifts.

Sir Black's Anniversary
This holiday is mentioned in the Devilgram The Rare Flower. It takes place in the month after Jill's Day and can also be celebrated with a tea party and gifts.

Helldown Day
Helldown Day is mentioned in the chats Helldown Day 1, 2, and 3. Helldown Day was created in the Devildom as a way to protect the balance of the three worlds. During the holiday, all demons must refrain from using their magic. The use of magical items is also prohibited.

Currency
The currency used in the Devildom is called Grimm. Grimm exists in the form of gold coins with a skull relief or brown banknotes. The player also uses this currency for various in-game expenses such as upgrading cards or unlocking spaces in Devil's Tree. Players can obtain Grimm by clearing battle stages or sending characters to work at D JOBS. Tasks and Pop Quiz events also often include Grimm as rewards.

Restaurants and Bakeries

 * AkuDonald's - a restaurant and parody of McDonald's mentioned in the chat Lord Diavolo's Fast Food Adventure
 * Café Lament
 * Cat's Eye
 * Déjà Vu - a fancy café
 * Demoning - a tea manufacturing company
 * Demon's Chocolate
 * Godevil Chocolatier
 * Headless Café
 * Hell's Burger
 * Hell's Kitchen - a restaurant and bar that's one of the eight job locations; Beelzebub often eats here
 * The Heinous Bee
 * Le Diable - a restaurant mentioned in the anime short
 * Le Pluvier - a restaurant mentioned in the anime short
 * Madam Devian's - a popular sweets shop
 * Madam Scream's - a popular bakery
 * Ristorante Six - an upscale restaurant and one of the eight job locations
 * Uncle Demon - a dessert shop where Beelzebub and Belphegor go to buy doughnuts

Retail

 * Akuzon Delivery Center - a shipping center and one of the eight job locations
 * "Akuzon" is a play on the enterprise Amazon and "akuma" (悪魔), which is Japanese for "devil"
 * Devilmart - a grocery store
 * Dogma Street - the biggest electronic district in the Devildom
 * Goetia - a hair salon Asmodeus mentions in the chat The Invitation
 * Hell Supermarket - a grocery store
 * Hocus Pocus - a shop that specializes in magical items and one of the eight job locations
 * Majolish - a clothing store that offers designer clothes and one of the eight job locations
 * "Majolish" comes from "majo" (魔女), the Japanese word for "witch"

Other Locations

 * The Demon Lord's Castle - Diavolo's residence
 * The Devil Planetarium
 * Diavolo's Private Beach - a private beach owned exclusively by Diavolo that has an artificial sun
 * The Fall - a popular nightclub Asmodeus frequents and one of the eight job locations
 * Hell Dome - a venue that holds events such as a sweets fair or a game festival
 * House of Lamentation - the seven demon brothers' dorm
 * MC moves in with them in Season 1 of the story
 * Lantern Night Inn - an inn visited by the brothers and MC in the event Let the Games Begin!
 * Makai Bank
 * "Makai" (魔界) is the Japanese version for Devildom
 * The Mausoleum - one of the eight job locations
 * Purgatory Hall - the dorm for exchange students
 * Simeon, Luke and Solomon live here during their stay in the Devildom
 * During Season 4 Raphael also stays here
 * The Reaper's Cave
 * The Royal Academy of Diavolo (RAD) - the Devildom's school
 * The Royal Library - one of the eight job locations
 * Siren Beach - a beach in the Devildom where Leviathan released Lotan
 * Witch's Retreat - a venue where Asmodeus plans to perform

Climate
In a text from Asmodeus, its alluded that there are four seasons in the Devildom despite the lack of a true day/night cycle. From this text, it can be assumed that the seasons function the same or at least similar to the four human world seasons, although currently (up to Lesson 16) there is no canon evidence of this aside from the text from Asmodeus.

In the Devilgram Shelter From the Rain, it rains in Devildom similar to the Human World. However, in the chat Suspicious Weather, there's acidic rain which Lucifer warns that humans should avoid.

Food and Drink
Eating in the Devildom is a sometimes dangerous experience, as some foods are too strong, too dangerous, or otherwise inedible for MC. The rest of the Student Council looks out for MC in this regard, but there is also mention of foods from the human world. It seems that there is some mixing of foods or foods inspired by the Human World.

Leaving the Devildom
There are several ways to leave the Devildom, as per Lesson 11-4. One way is making use of one of several "external magic seals" that work as passages between worlds. All demons aside from Lord Diavolo, Barbatos, and Lucifer must receive "permission in advance" before they can use the seals. This is due to the previous misuse of their free availability. It is strongly implied that Mammon was the perpetrator.

Another method, considered the "classic route", involves an individual walking through all eight layers of the Devildom on foot. However, it is considered "too long" and, as Lucifer mentions, would destroy MC's psyche.

Witches or sorcerers can also summon individuals out of the Devildom, though there are alluded consequences to this method.