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.""

- Ramiris

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 S simply for being members of the council.

When the time comes for the birth of a new Demon Lord, the other Demon Lords will not stay silent. If a fool freely calls himself a Demon Lord, he or she will be killed because they would have invoked the Demon Lords' wrath. However, if the said person could withstand the wrath of a Demon Lord, they themselves would be recognized as such.

But, a Demon Lord with such ability was not a common occurrence. Therefore, for a stranger to claim himself as a Demon Lord, it was necessary to get 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 The Octagram through Rimuru's suggestion mainly due to Clayman's death and Frey's and Carrion's announcement of stepping down and becoming Milim's subordinates, as well as the awakening of Rimuru as a True Demon Lord. Thus, resulting in them being reduced to 8 Demon Lords.

Walpurgis Banquet
From time to time the Demon Lord Council gathers together 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, building either nations or have subordinates that built it for them. While not all of them are protective of their homes, some will be immediately hostile should another Demon Lord encroach uninvited into their homes, especially if such invasion involves destruction.

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

Millim Nava: Annexed territories of The Capital of the Forgotten Dragon, Beast Kingdom Eurazania, Harpy Country Fulbrosia and Land of Puppets Jistarv (southeast territories at the center of the continent).

Ramiris: Formerly in an isolated part of the Kingdom of Ulgrassia, now residing within the Jura Tempest Federation inside her Labyrinth.

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 Ramiris' Labyrinth. 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; Demon
 * Milim Nava - True Demon Lord; Dragonoid
 * Ramiris - Fairy Queen; Fairy
 * Dagruel- Titan; Giant
 * Roy Valentin (Role as a substitute for Luminous Valentine Deceased) - Bloody Lord; Vampire
 * Dino - True Demon Lord; Fallen Angel
 * Kagali (Replaced by Leon)
 * Carrion - Lion King; Lycanthrope (Became Milim subordinate)
 * Frey - Sky Queen; Harpy (Became Milim subordinate)
 * Clayman (Replaced by Rimuru) - Marionette Master; Deathman
 * Leon Cromwell - Ex-Chosen Hero; Ex-Human

Trivia

 * The previous Ten Great Demon Lords took about 3 months to decide on a name. During that time the humans already named them without their permission in the first place.
 * Rimuru only took 3 minutes to decide on a name for the current Demon Lords. (Octagram).
 * As stated by Kagali, "Curse Lord Kagali," over 200 years prior to Rimuru's own admission into the group, the Council of Demon Lords once only consisted of Seven Members during the time of the creation of the official rules of the Walpurgis Council and the three majority vote rule.
 * Due to the True Demon Lords gradually lessening their activities over the millennia. A majority of the world at large believe that the overall power of a Demon Lord to only be as powerful as a Demon Lord Seed due to no being having successfully ascended into one for the last few thousand years and the more active members of the Council being Carrion, Frey, Clayman, and Valentin, who only reached the level of a Demon Lord Seed. Causing individuals such as Mariabell Rosso to deem themselves powerful enough to fight and win against an actual awakened True Demon Lord, as seen where Mariabell underestimated Rimuru due to deeming that her power was superior to his by using Carrion as a benchmark.
 * According to Guy, the "first generation" of Demon Lords was him, Millim and Ramiris; the "second generation" were Dagruel, Luminous and Dino and the "third generation" consisted only of Leon. He doesn't count those that died (Such as Kazaream) over the years during the Great Tenma Wars or other incidents.