Holy War

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 humans. It serves as a catalyst for most of the series.

Overview
Over 3,000 years ago, the Goddess Clan and Demon Clan constantly clashed with each other, but due to both 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 Demon King 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 Supreme Deity 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.

Outcome
How the war ended is not explained, but it is revealed 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 the material 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.

New 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 her mother 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.