"It is an evil thing that brings together seven demon kings. According to a legend that only a few people know, in ancient times, there existed a giant dragon called "Tasamat", who was the original incarnation of all evil. In the legend, it was torn in half in an epic battle with the creator god "Anu", and the Burning Hell was transformed from its corpse."
- Tyrael

Tasamat
I believe that old players of "Diablo 2" must still remember the three bosses in the last three acts. They are Mephisto, Diavolo and Baal. In the plot dialogue of the game, the relationship between the three demon kings has been clearly introduced: Mephisto is the eldest brother, the second brother is Baal, and Diavolo, highlighted on the game cover, is the third of the three brothers. However, even after beating the game, many players are still confused about the origins of the Three Demon Kings. In fact, whether it is Andariel and Duriel in the first and second acts, or the three demon kings in the later levels, they all come from the same father, and the source of evil is the character to be introduced in this article: Tasamat.
Character Description
Tasamat is the second entity born in the world after the creation god "Anu". Tasamat was Anu's eternal enemy, and its death led to the formation of the Burning Hell.
The origin of evil
"The seven demon kings were born from the seven heads of Tasamat. The seven hell realms were formed from its body. From then on, the endless spread and corruption began, and the world fell into a Cycle after cycle. "
- Adelia

The seven-headed evil dragon
The same person is mentioned in various myths and legends: Anu. It was the first entity to exist in the world, and before Tasamat appeared, Anu was also the only creature with autonomous consciousness in this world. In order to achieve a state of complete sanctity, Anu stripped his evil self from his body. However, these detached evil things did not scatter and gradually disintegrate. Instead, they combined into a new entity in the world: Tasamat, a giant dragon with seven heads, which is considered the source of all evil. Although Tasamat has become another entity independent of Anu, it and Anu are still trapped in the pearl of the original formation of the world. For hundreds of years in the world of pearls, they have been engaged in endless battles and struggles. In the final battle, Tasamat and Anu died together. After Tasamat died, its black and scorching body spiraled through the air and fell into the abyss of the lower world. From its decaying flesh were born the seven realms of the Burning Hell. The maggots eating away at its flesh turned into a race of demons living in the seven realms. And each of its seven heads became a demon king who ruled one of the seven realms. King Baal of Destruction, King of Hatred Mephisto, and King of Fear Diavolo were born from the three most powerful heads of Tasamat, and its four lesser heads became the later King of Sin, Azmodan, King of Lies, Belial, Queen of Pain, and Duriel, the Queen of Pain.
Rebirth of the Dragon
"That means Diavolo wants to become an evil dragon like Tasamat again?"
"From a certain To a certain extent, yes. ”
——The conversation between Nephalem and Tyrael

The battle between heaven and hell
After countless years, Tasamat has returned to the world in a sense.Diavolo, also known as the Lord of Fear, transformed himself into a unique demon by condensing the entities of six other demon lords into his own body. When Diavolo was defeated by the Nephalem in heaven and his soul was sealed into the Dark Soul Stone, his entity gradually merged with the entities of the other six demon kings. At the moment when Malthael, the Angel of Death, shattered the Dark Soul Stone, the seven-in-one Diavolo, like the ancient Tasamat, was reborn in the world.
The inspiration for the character

Three Brothers of Hell
The name Tasamat can be taken from Tiamat in Babylonian mythology, who is a sea goddess in the form of a dragon in mythology. She combined with Abzu, the god of fresh water, and gave birth to the gods. This is very similar to the story of Tasamat and Anu creating the universe in the conflict in the game. Tiamat was later killed by Marduk, and her broken body formed the world, just like Tasamat's body turned into the burning hell in the game setting. In Dungeons and Dragons, Tiamat is often depicted as a five-headed dragon.
As for the form of Tasamat, it is more likely that it was influenced by the monsters described in the New Testament. In some myths of the Middle East and Europe, the seven-headed dragon is often depicted as an evil creature that destroys the world.