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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' 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 three or more Demon Lords. As if you messed with the new Demon Lord, the supporting Demon Lords will become opponents at the same time.

To become a 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 sacrifices 10.000 Human being (souls).

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 Holy Demon 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
After an individual transforms into a True Demon Lord the “Harvest Festival” commences. It is the festival that accompanies the evolution. Upon its completion, related monsters are blessed with a gift.

The Harvest Festival is a festival where a Demon Lord evolves into a True Demon Lord. Among the 10 Demon Lords, only 4 are True Demon Lords. Evolving into one is, for a Demon lord, as difficult as a normal person to evolving into a Demon Lord.

Even those who have lived for many eons are not guaranteed such status. Orc Disaster would have become one, but was stopped in the middle of his transformation. Thus, the Demon Lords watch each other, in the hope of preventing others from evolving above them.

Current Demon Lords

 * Ramiris
 * Ruminas Valentine
 * Dino
 * Leon Cromwell


 * Guy Crimson - True Demon Lord
 * Milim Nava - True Demon Lord
 * Dagrule - True Demon Lord
 * Rimuru Tempest - True Demon Lord

Previous Demon Lords

 * Clayman†
 * Kazaream†


 * Karion - Stepped down, currently Milim's subordinate
 * Frey - Stepped down, currently Milim's subordinate

Informal Demon Lords

 * Orc Disaster Gelmudo†
 * Diablo - Demon Duke

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. It was an unprecedented treatment.

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

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

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 officials after destroying the latter one. And, he permitted them to continue serving him.

Milim awakened as a 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 power. 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.

World War
This was a turbulent event where new Demon Lords replaced the old. This event would occur once every 500 years. 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.

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.