Octagram

"In other words the Walpurgis Banquet is just the Demon Lords gathering together to update each other on each other’s status while discussing trending topics. It’s just that the humans certainly wouldn’t know about that, but it’s really nothing dramatic."

- Lamrys

Octagram (八星魔王) refers to an exceptionally powerful group of existences in the Tensei Shitara Slime Datta Ken universe. Although few have been active, they have all been ranked at least Disaster simply for being members of the council.

When the time comes for the birth of a new Demon Lord, other Demon Lords will not stay silent. If a fool freely calls themself a Demon Lord, they will be killed for invoking the Demon Lords' wrath. However, if said person could withstand the wrath of a Demon Lord, they themselves would be recognized as such. But that did not tend to be a common occurrence.

For a stranger to claim the title Demon Lord, it was necessary to gain the support of at least two or more Demon Lords – as if you messed with the new Demon Lord, the supporting Demon Lords will become opponents at the same time.

There is a non-aggression pact between the Demon Lords. However, if one initiates a fight, they may gain the wrath of all the other Demon Lords.

The council had many different names in the past, the second-most recent being The Ten Great Demon Lords which was later changed by the Demon Lords to Octagram through Rimuru's suggestion, mainly due to Clayman's death and Frey and Carrion stepping down and becoming Milim's subordinates, as well as Rimuru being recognized as a member of the council, resulting in their ranks being reduced to eight.

Walpurgis Banquet
From time to time, the Demon Lord Council would convene in what is known as the Walpurgis Banquet. This meeting takes place in a separate dimensional space and is supposed to handle serious issues, however, very often the members of the council just use it as an excuse to hang around and have a tea party, unbeknownst to mankind, who view every instance of Walpurgis as a precursor to a worldwide war or another Tenma Great war.

Territory
All Demon Lords tend to have a territory they call their own. While not all of them are protective of their homes, some will be immediately hostile should another Demon Lord encroach uninvited, especially if such invasion involves destruction. Areas outside the designated domains do not fall under a Demon Lord's reign unless approved by the Council or the inhabitants wish to serve a Demon Lord.

Rimuru Tempest: Jura Tempest Federation (Great Jura Forest at the center of the continent).

Milim Nava: Annexed territories of The City of the Forgotten Dragon, Beast Kingdom Eurazania, Harpy Kingdom Fulbrosia and Puppet Nation Jistav (southeast territories at the center of the continent).

Lamrys: Formerly in an isolated part of the Ulgrasia Republic, now residing within the Jura Tempest Federation inside The Dungeon.

Guy Crimson: Northern Continent.

Leon Cromwell: El Dorado (Southern Continent).

Luminous Valentine: Holy Empire Ruberios (Western Nations are partially part of her territory as well).

Dino: Formerly residing in the Barren Lands with Dagruel, now in the Jura Tempest Federation within The Dungeon. He is the only Demon Lord with no territory or subordinates to call his own, as such he tends to drift between his fellows' territories to "mooch" from them.

Dagruel: Barren Lands (Westernmost part of the central continent).

Ten Great Demon Lords (Defunct)

 * Guy Crimson - True Demon Lord; Daemon
 * Milim Nava - True Demon Lord; Dragonoid
 * Lamrys - Fairy Queen; Pixie
 * Dagruel- Titan; Giant
 * Roy Valentin (Role as a substitute for Luminous Valentine Deceased) - Bloody Lord; Vampire
 * Dino - True Demon Lord; Fallen
 * Kagali - Curse Lord; Deathman (Replaced by Leon)
 * Carrion - Lion King; Lycanthrope (Became Milim subordinate)
 * Frey - Sky Queen; Harpy (Became Milim subordinate)
 * Clayman - Marionette Master; Deathman (Replaced by Rimuru)
 * Leon Cromwell - Ex-Hero; Demonoid

Trivia

 * Guy is the de facto leader of the group, due to being a powerful Primordial and the first Demon Lord.
 * The previous Ten Great Demon Lords took about 3 months to decide on a name. That ended in naught as the masses already named them without their permission.
 * Rimuru only took 3 minutes to decide on a name for the current Demon Lords (Octagram), making Dino wonder why they tormented themselves over the name; Rimuru was shocked by their lack of creativity.
 * As stated by Kagali, over 200 years prior to Rimuru's own admission into the group, the Council of Demon Lords once only consisted of seven members prior to the creation of some of the official rules of the Walpurgis Council, including the three majority vote rule.
 * This was to lessen the damage and loss of life that would occur when two Demon Lords fought; Kagali fought Roy Valentin repeatedly, resulting in intervention (likely by Guy).
 * As the True Demon Lords have gradually diminished their activities over the millenniums, a majority of the world believes that a Demon Lord's overall power is equivalent to a Demon Lord Seed because no being has succeeded in ascending to one for a few thousand years. Carrion, Frey, Clayman, and Valentin, who are the most active members of the Council, have only reached the level of a Demon Lord Seed. Due to this, individuals such as Mariabell Rosso thought they were powerful enough to fight and win against an actual awakened True Demon Lord, as shown in the scene where Mariabell underestimated Rimuru by comparing her power with Carrion's.
 * According to Guy, the "first generation" of Demon Lords was him, Millim and Lamrys; the "second generation" was Dagruel, Luminous and Dino and the "third generation" consisted only of Leon. He doesn't count those that died (Such as Kagali) over the years during the Great Tenma Wars or other incidents.
 * Of the Octagram, Lamrys, Dino and Milim tend to spend their time in Rimuru's territory. Guy, Leon and Dragruel have no interest in Jura, while Luminous wishes to stay away from Veldora.
 * Milim boasts the largest domain out of all the Demon Lords (a combined four territories), with Rimuru coming in second with the Jura Forest.
 * True Demon Lords don't have to join this organization, otherwise it would have 22 members after Rimuru evolved his Twelve Guardian Lords; plus Rain and Misery.
 * On this note, Guy will investigate if numerous awakened Demon Lord presences emerge, as shown with him questioning Rimuru. This led to him bargaining for his maids' evolution in exchange for putting the matter aside.