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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, all of whom were not True Demon Lords, didn't possess any Ultimate Skills and had fairly weak armies compared to their fellows.

Ruminas and Leon Cromwell both appear to possess a vast amounts of personal power and are possessors of Ultimate Skills, the strength of their armies are however vastly different. Specifically while Ruminas controls 7 Vampire Dukes who are themselves Demon Lord Class Individuals, 7 Sages, who can match unawakened Demon Lords, 400 A+ Rank Bloody Knights, the weakest of whom are on par with the Knights Templar, which, alongside the entire Western Saints Church, are also under her control as her face to the public alongside the entire Holy Empire of Ruberious. This means that all of the countries that follow that religion and all the elite forces included are also indirectly under her control. It's no exaggeration to say everything west of the Great Jura Forest can be considered part of her domain.

Meanwhile 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 control over at least 10.000 A- Ranked beings and his strongest six Leader Knight Corps members stand at the Level of named Arch Demons. With this much power together with his own True Hero status it's no surprise that he gained an entire country and continent to reign over. Furthermore he has established powerful defences for his territory.

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 seems to be the greatest military might seen out of the Ten Great Demon Lords with a True Dragon, Velzard, two Demon Nobles, at Demon Duke hierarchy, making them stronger than any unawakened Demon Lord, alongside several thousand demons all following him. Yet despite being in possession of such a frightening army by his own words anyone below True Demon Lord level is simply of no use to him, and Ultimate Skills aren't even a concern. He only really seems to see Velzard as a usable war potential.

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, increasing her already vast military power.

Milim has meanwhile left the command of her whole army to Karion, who took the position of general and received her personal training, far surpassing his previous strength. This army stands at 300,000 strong and at the center lay the “Flying Beast Knight Order”, which consists of 3,000 griffons in service to “Sky Queen” Frey’s mounted upon whom sit specially picked beastmen from “Beast King” Karion’s former ranks. The warriors of this order all exceed A Rank in power and the entire force is easily able to move as one effective body. The Griffons were originally magic beasts of only B+ Rank at best, but were raised to A- Rank through training given by Karion. The rest of Milim's army consists of devils, mercenaries and Clayman’s former subordinates. In times of war even the people who are usually managing public order can be roped in to bolster the ranks further.

The largest influence on military power in the Eight Star Demon Lords however is the newcomer, Rimuru Tempest, who has gathered a True Dragon, Veldora, 12 True Demon Lords, 7 Demon Nobles, several hundred elite Arch Demons and every race in the Great Jura Forest under his rule by the time the group was ready to fight the Angels. Rimuru's country, Tempest has since become the unofficial focus of the Demon Lord's gathered might. They have also formed alliances and trade agreements with Sarion, Dwargon, Holy Empire Ruberios, The Western Saints Church, Eldorado, and the unnamed lands of Demon Lord Milim. Furthermore Demon Lord Dagrule also sent his children to learn in Tempest while Dino came to work there (and spy on them for Guy and Yuuki).

Another factor however is the presence of a second Demon Lord, Ramiris, among the eight joining with his county, Tempest, and providing him the services of her dungeon. Through mutual aid, Ramiris gained powerful subordinates including 4 Dragon Kings, provided by Milim, and Baretta the battle doll. In return she provided a near impregnable fortress full of monsters that can never die so long as she lives, and the walls can trap and kill off hostile invaders at her will by transporting them to any random floors. After all of this, Ultimate Skills became commonplace amongst the leadership and 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.

Perhaps the greatest strength of this new generation is how there's little infighting and a large amount of co-operation. For example Rimuru awoke Guy's two Demon Dukes into True Demon Lords, further strengthening Guy's Forces, while Ramiris's dungeon provided Ruminas' Templars with a suitable training ground. Furthermore several of Guy's subordinates sacrificed themselves to empower the six elite knights serving Leon to old Demon Lord standards, while Rimuru would later help Leon himself become a Demigod. Indeed during the Tenma War Tempest frequently sent out its cabinet members to support their allies.

The Eastern Empire was most shocked by this drastic increase in war potential. While they were justified in believing that they could wipe out the armies of three demon lords (Karion, Frey and Clayman for example) and they believed themselves powerful enough to defeat Ruminas's forces, they found themselves being completely crushed by Tempest, and in the process provided them the souls needed to create those 12 True Demon Lords stated above. The fallen empire whould then see its territory swallowed by Tempest.

Eventually Dino and Dagrule would betray the Eight Star Demon Lords in favor of the angels, thus reducing the actual number of active lords to six during the Tenma War. That said, Dino was willing to spy for Tempest following his defeat.

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.

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).