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The Eight Star Demon Lords are exceptionally powerful existences in the Tensei Shitara Slime datta ken universe. Although few have been active, they have all been ranked at S simply due to their title.

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 invoked the Demon Lords's wrath. However, if 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.

Since the Demon Lords exercised restraint, the humans buy them off by handing over fertile land.

Though they are called the ten great demon lords, that was a name the humans selfishly decided for them. But, the Tenma Great War 500 years ago left less than ten of them. And then, the new Demon Lords suddenly decided to fight for dominance and decided to keep the number at ten. So this rule is more of an unspoken agreement, if anything.

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.

True Demon Lords
To become a True Demon Lord and getting the support simultaneously, one must fulfill the condition for evolving into the Demon Lord (from a seed to a sprout), it is necessary to sacrifice 10,000 souls, while subordinates of a true demon lord need 100,000 Human being (souls) to evolve into true demon lords themselves as shown in Rimuru and Guy Crimson's situation.

Among the 10 Demon Lords there were only 4 True Demon Lords. Evolving into one is, for a Demon Lord, as difficult as a normal person to evolving into a Demon Lord.

The Ten Great Demon Lords Era
The Ten Great Demon Lords are an old standard that are currently still employed by the Eastern Empire, however they were much weaker than the current Eight Star Demon Lords are. Guy Crimson frequently allowed the Demon Lords to fight amongst themselves in order to strengthen their faction against the Eastern Empire and the Army of Heaven, meaning there has been a lot of infighting amongst them.

The original Demon Lords, Guy, Milim and Ramiris (In her adult form) were by far the strongest and are known as the old generation, it is noted that Dino and Dagrule are very powerful individuals as well with the latter having even fought against Veldora, a True Dragon.

The weakest demon lords out of the Ten Great Demon Lords appears to have been Clayman, Frey, Kazaream and Karion, who were not True Demon Lords, doesn't possessed Ultimate Skills and had fairly weak armies compared to their fellows.

Ruminas and Leon Cromwell both appear to possess a vast amount of personal power and are possessors of Ultimate Skills, the strength of their armies are however vastly different. Specifically while Ruminas controls 7 Vampire Duke who are Demon Lord Class Individual,7 Sages, 400 A+ Rank Bloody Knight which the weakest of them on par with Knights Templar, the entire Western Saints Church, Holy Empire Ruberious, all of the countries that follow that religion and all the elite forces included, While Leon merely allowed those who wished to be ruled by him do as they please and has no expectations of them. Even so Leon has Controls 10.000 A- Rank and his Strongest 6 Leader Knight Corps at the Level of Named Arch Demon and he gained an entire country and continent to reign over and has established powerful defences for it.

Milim and Dino took no interest in military power and relied solely on their personal might to maintain their positions. Meanwhile Guy Crimson had what had to be the greatest military might seen out of the Ten Great Demon Lords, with a True Dragon, Velzard, two Demon Nobles, alongside several thousand demons following him.

The Eight Star Demon Lord Era
The current Eight Star Demon Lords seem to possess a higher standard of military power than the previous incarnation. Demon Lords Karion and Frey felt they were too weak and became subordinates of Milim, in turn raising the bar of what a Demon Lord's personal power must be, while Ruminas, though the Western Saints Church and Hinata, has gained indirect control over the Freedom Association and its adventurers.

Milim’s currently assembled troops have been assembled through the reorganization of the forces of her area. The command of the whole army is left to Karion who took the position of general. Their total number is 300.000. At the center of them was the 3.000 units titled “Flying Beast Knight Order”. 3.000 griffons of “Sky Queen” Frey’s forces and mounted on their back are the specially picked beastmen of “Beast King” Karion’s subordinates. They’re merely 3.000 people but they are warriors who exceed A rank who are able to easily do high speed air battle as one effective body. In this world, this is the largest force composed only of people who exceed A rank. Even the Empire’s Magic Beast Corps was composed of “A-“ rank, so it could be easily to comprehend that they are a significant forces. Griffons are originally magic beasts with “B+” rank. But, with the result of the training given by Karion, their ability was raised to “A-“ rank. As with the A rank beastmen who rode them, their ability became equivalent to A rank. The Flying Beast Knight Order is the strongest subordinates who were trained by Karion in both name and reality. The rest are the devils, mercenaries and Clayman’s former subordinates. It is a combined force consisting of various people. Even the people who usually take part in maintaining the public order were roped into the war.

The largest influence on military power in the Eight Star Demon Lords however is the newcomer, Rimuru Tempest, who had gathered a True Dragon, Veldora, 12 True Demon Lord, 7 Demon Noble, several hundred elite Arch Demon and elite beings under his rule by the time the group was revised into its current form. Rimuru's country, Tempest has since become the unofficial focus of the Demon Lord's gathered might, as by the time the Eastern Empire waged war with them they'd formed alliances and trade agreements, with Sarion, Dwargon, Ruminas, The Western Saints Church, Leon Cromwell, Milim and several other True Dragon countries while Demon Lord Dagrule sent his children to learn in Tempest while Dino came to work there (and spy on them for Guy and Yuuki).

Tempest's personal power after preparing for Tenma Great war is also notable having 12 True Demon Lord, 7 Demon Noble class combatants and a roughly sixty thousand man strong army of evolved monsters. Ultimate Skills became commonplace amongst the leadership. Their might was such that even Guy was shocked enough to make an inquiry and allied with them after learning they had no ties to his enemies. Rimuru also awoke Guy's two Demon Dukes into True Demon Lords, further strengthening Guy's Forces.

The Eastern Empire was most shocked by this war potential, as they were using the standards of the, now defunct, Ten Great Demon Lords, which were far weaker. For example the empire is justified in believing they could wipe out the armies of three demon lords (Karion, Frey and Clayman for example) and they believe themselves powerful enough to defeat Ruminas's forces, but Tempest, when compared, has become the strongest individual faction, not counting leaders as Guy and Milim are in a league of their own, and is incomparably superior to the other's, who are themselves all superior to those four factions stated above, one for having absorbed two of them into itself.

Ten Great Demon Lords (Defunct)

 * Guy Crimson - True Demon Lord
 * Milim Nava - True Demon Lord
 * Ramiris - Fairy Queen
 * Dagrule - True Demon Lord


 * Ruminas Valentine - Vampire Princess
 * Dino - Fallen Angel
 * Kazaream† (Replaced by Leon)
 * Karion - Stepped down, currently Milim's subordinate
 * Frey - Stepped down, currently Milim's subordinate
 * Clayman† (Replaced by Rimuru)
 * Leon Cromwell - True Hero

Eight Star Demon Lords (Current)

 * Guy Crimson - True Demon Lord
 * Milim Nava - True Demon Lord
 * Ramiris - Fairy Queen
 * Dagrule - True Demon Lord
 * Ruminas Valentine - Vampire Princess
 * Dino - Fallen Angel
 * Leon Cromwell - True Hero
 * Rimuru Tempest - Great Demon Lord

Eight Star Demon Lord Domains
Compared to human cities and villages the magic energy density is higher, but not at levels at which humans can’t survive. Even if it is called a Demon Lord’s dominion, unless we are talking about their residence or perhaps their province, normal people can also live there. As long as they pay the demon lord the appropriate tax, their safety is guaranteed. There are also demon lords with unknown dominions.

Currently, Rimuru’s territory consists of the entirety of the Great Jura Forest and the Eastern Empire.

Frey ,Karion as well Clayman’s territory were merged and put under the rule of Milim. later The Heaven will be included as her Territory too. In the end, her rule is nominal. Territory management will be left to Karion, Frey, Dino as well as Milim’s people.

Guy's territory consists of the Ice Continent. While Ramiris reigns over the Dwelling of the Spirit.

Ruminas's Territory consist of the Holy Empire Ruberious.

Leon's territory consist of the Magic Country El Dorado.

Dagrule's territory Consist of the Giant Kingdom.

History
Guy granted the wish of his powerless summoner and destroyed an enemy nation. As payment, he destroyed the summoner’s country. And with this, though it was unnecessary, he noticed his awakening as a True Demon Lord. The two Arch Demons he summoned to destroy the first country evolved into Demon Duke after destroying the latter one. And, he permitted them to continue serving him.

Milim awakened as a True Demon Lord at the same time as Guy. Her pet dragon was killed by some country. In her anger, Milim destroyed the country and awakened as a True Demon Lord. Unconscious, she fought with Guy. That battle continued for seven days and seven nights, transforming the bountiful lands in the west into a wasteland.

Eventually, the battle concluded. Milim regained her consciousness and the battle ended. The one who returned her sanity was Ramiris. In the process, the ruler of spirits Ramiris absorbed the evil magic of demons and the powerful aura of dragons and began to change. But, she was able to stop Milim’s rampage. And the three settled this dispute.

These three were the first demon lords. The decided that each of the three would be different than the other two. One would pursue the Ultimate Skill. One would live as one pleased. One would judge the world. Because their objectives were different, they could recognize each other.

Later, the protector of Heaven’s Gates the Giant, Dagrule, the Ancient Vampire, Ruminas Valentine, and the being who fell from heaven itself, Dino, increased their numbers to six. They were the second generation, demon lords inferior to the oldest.

Dagrule due to holy power ingrained in his body, rejected the Demon Lord Seed. But, due to the many wars the demon lords started to eliminate the giants and the fairies, he awakened as a True Demon Lord. Even so, he has yet to acquire an Ultimate Skill that Guy and Milim possess.

Rimuru --> Shion and 100 Goblin Died ---> Rimuru killed 15.000 Troops ---> He evolve into True Demon Lord --> Kill Clayman ---> The end.

Holy Demon Great War (Known as Tenma Great War)
This was a turbulent event where new Demon Lords replaced the old. This event would occur once every 500 years and the fight took around a Week. At the same time as Karion three other Demon Lords were born. Frey is one of those. Considered the youngest, with least experience in the war, are Demon Lord Karion, and the last to be born, Demon Lord Leon Cromwell. The six new Demon Lords were thus the members of the new generation. The old generation, on the other hand, has survived over two such wars; their strength far surpassing the rest. Therefore, most of the new generation Demon Lords strive to expand their power.

The truth those Angel is come from one of Ultimate Skill Justice King Michael's Ability Armageddon which summon a Million Angel and 13 Seraphim which have the same Magical Amount as True Demon Lord.

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 of the current Demon Lords.(Eight Star Demon Lords).