Holy War「聖戦, Seisen」was a huge conflict that took place 3,000 years ago between the Demon Clan and the Stigma alliance of the Goddess Clan, Fairy Clan, Giant Clan, and eventually the humans. It serves as a catalyst for most of the series.
Over 3,000 years ago, the Goddess Clan and Demon Clan constantly clashed with each other due to bitter rivalry between the Demon King and Supreme Deity, but due to both races being so powerfully equal, they sustained a balance of power among the five races. Eventually that balance was broken when Meliodas, the leader of the Demon Clan's Ten Commandments and the Demon King's eldest son, betrayed his clan by killing two members of the Ten Commandments and fled with his lover, the Goddess Elizabeth.
This moment of weakness among the Demon Clan gave a huge opening for the Goddesses, and took the opportunity to ally with the other races and strike against the demons, starting the Holy War. While most of the war is unknown, thousands from all of the clans lost their lives in the war.
Nearing the end of the war, Meliodas and Elizabeth's parents were fed up with their children's betrayal against their clans and punished them by killing them and giving them each a unique curse. The Supreme Deity punished Meliodas with a curse that would cause him to stop aging and bring him back to life and devour his emotions every time he dies without fail, while the Demon King cursed Elizabeth by making her reincarnate as a human and every time she dies she would forget all memories of her past lives, and if she were to regain all memories of her past lives, she would inevitably die three days later in front of Meliodas.
How the war ended is never truly explained, but it is said to have happened during the time between Meliodas' first death and resurrection: the outcome being the Demon Clan having been sealed away in a ritual done through the Coffin of Eternal Darkness, bringing peace to Britannia for the next 3,000 years. The Goddess Clan sacrificed their physical forms to seal away the demons and their souls came to inhabit different relics found in Britannia, while the giants and fairies withdrew to their own realms, leaving the humans as the dominant race in Britannia, causing them to spread and populate the land and dividing themselves into different kingdoms. It was stated by Ludociel that the Goddesses had to seal the Demons after their strongest warrior, Mael of the Four Archangels, was slain by Estarossa of the Ten Commandments but the truth was that Gowther had manipulated the Goddesses to end the Holy War by, with help from his doll, using a forbidden spell that would sacrifice his life in the process. The spell altered everyone's memories of Mael - even the gods themselves - by thinking he was killed by the Demon King's supposed middle son Estarossa, whom Mael would take on that persona.
Second Holy War
3,000 years after the original war, the Ten Commandments were freed from the Goddesses' seal and began wreaking havoc among Britannia and soon were able to take over the majority of the land. After Meliodas reveals the truth behind his and Elizabeth's past, Elizabeth's memories are returned by Zeldris and the curse placed on her begins to take effect. During this time, the Seven Deadly Sins plan to take back the kingdom of Camelot from the Demon Clan but the Ten Commandments place a trap, ultimately forcing Meliodas to revert back to his original self.
As this occurs, the Goddesses begin to take the physical forms of Humans to serve as vessels for their disembodied souls. The former leader of Stigma: Ludociel, possesses Margaret's body along with his fellow Goddesses: Sariel and Tarmiel. They meet up with Elizabeth and the Sins (except Meliodas, Ban, and Gowther) back in Liones, where Ludociel makes an agreement with Elizabeth and the Sins to reform the Stigma Alliance to fight the Demon Clan and start a second Holy War.
The war involved many battles between the allied forces and the forces of the Demon Clan. Liones is defended by several of the Holy Knights that formed the "Defensive Force". The Fairy King's Forest is defended by the armies of the Fairy and Giant Clan, who later join the "Search-and-Destroy Force" to fight the largest troops of the demon army.
Estarossa of the Ten Commandments who is later revealed to be Mael of the Four Archangels, is consumed by the power of the Commandments that he had absorbed due to his rage towards Gowther for using and torturing his mind. He is confronted by several of the Diane, King, Gowther, Elizabeth Liones, two of the Four Archangels and Derieri; the last three being killed in the course of the battle. Mael is finally defeated by the Fairy King and returned to his former self by Gowther, expelling the Commandments from his body.
With Camelot completely taken over after the death of Arthur Pendragon, the "Assault Force" is directly confronted with the headwaters of the Demon Clan: Zeldris, Cusack and Chandler. After a hard battle, the two major demons decide to merge in their true form, the Original Demon. To make matters worse, the five Commandments that were free, meet to be absorbed by Meliodas, who already had in his possession the other five and was in the process of becoming the new Demon King.
The Assault Force looks unconscionable while Escanor is defeated by the Original Demon and Merlin is immobilized to carry out the Chrono Coffin that will hang the cocoon of Meliodas in time before becoming the Demon King. However, with the help of the Sins, Elizabeth and Mael, Zeldris and the Original Demon are finally defeated and the Chrono Coffin is completed.
However, Meliodas emerges from the cocoon before the time was suspended, revealing himself as the new vessel of the Demon King. The Sins, as well as Ludociel and Mael try to fight, but the Demon King proves to be too powerful even for their combined strength. Before the Demon King can kill Hawk, Ban appears to save him and fight the Demon King hand-to-hand due to his time in Purgatory while Meliodas confronts his father in the mindscape but is overwhelmed by his father and nearly loses the fight until Elizabeth and the other Sins enter his mindscape to encourge him and help him to fight against his father's control. Through the efforts of Meliodas and Ban, the Demon KIng is forced out of the former's body and defeated.
With the defeat of the Demon King, the remaining troops of the Demon Clan retreat from Britannia, officially ending the Holy War, at least for the time being due to the racial tensions between the Demons and the other races.
Aftermath of the Second War
Later that night, after Meliodas breaks the curse on him and Elizabeth, the Sins return to Liones where they hold a victory party in the Boar's Hat where everyone who survived the war gathered to celebrate the war's end. The next day, the Seven Deadly Sins, Hawk, and Elizabeth go out on a supply run but it is an excuse for Meliodas to return to the remains of Edinburgh so he can undo the seal on Gelda that he placed twelve years ago.
Upon seeing it is Meliodas, and not Zeldris, Gelda accepts that she will likely never see Zeldris again but Meliodas tells her she can't give up and she goes off to fly him, despite Meliodas claiming he's likely no longer alive but Gelda says that if it's true, she will join him in the next world regardless.
Back at Liones, Bartra summons everyone to announce that he wishes for Meliodas to marry Elizabeth to become the new king due to his growing age but Elizabeth herself says she can't, as she intends to go with Meliodas to the Demon Realm, shocking everyone except for Merlin.
The next morning, Meliodas disbands the Seven Deadly Sins as he and Elizabeth make their way to the portal to the Demon Realm where Meliodas waves goodbye to everyone, but as soon as he does the giant rock next to them suddenly breaks apart lands directly on Elizabeth, supposedly crushing her to death which leaves everyone horrified and Meliodas freaks out. Shortly after, it's revealed that Merlin teleported Elizabeth away at the last second, saving her life.
The cause of the "accident" is that Elizabeth's curse has somehow returned and attempted to kill her in front of Meliodas, but failed due to Merlin's intervention. Meliodas makes sure to get rid of the curse for good, but to everyone's confusion, the curse keeps regenerating itself with Meliodas and Ban both realizing that the Demon King is still alive with Meliodas himself further realizing, to his fear and frustration, that Zeldris has become his new vessel, meaning the Holy War has not yet ended.
The resurrected Demon King goes to Lake Salisbury in order to regain the magic power he lost in his previous encounter with the Sins, and to distract them meanwhile, he summons an Indura from the Demon Realm. While the rest of the Sins besides Escanor fights the Indura, Meliodas and Elizabeth confronts the Demon King. Meliodas manages to dominate the battle even without having to use his godly power but the Demon King realizes this is because Meliodas is afraid of killing Zeldris' body if he does and that he can't kill him as long as he is in Zeldris' body which he uses to full advantage as his power continues to replenish.
It is not until the rest of the Sins (including Escanor) join the battle that Meliodas, with the help of Gowther and Gelda, manages to make Zeldris face the Demon King in Zeldris' mindscape. With the defeat of the Demon King in both the real world and the mindscape, Zeldris regains his body and the Demon King is forced to take drastic measures to defeat his enemies by taking over the land of Britannia itself as his new vessel, despite it being incompatible.
The Seven Deadly Sins each use their strongest techniques combined with Meliodas repeatedly using Full Counter on the magic energy to amplify it over and over again. Seeing through his father's false pleas of mercy, Meliodas is able to unleash their ultimate combination technique: Nemesis, onto him and destroys his physical form; but in order to truly prevent the Demon King from returning, Meliodas sacrifices his godly power to cancel out the Commandments, destroying them forever, thus ending the Holy War once and for all.