Holy War

The Holy War was a huge conflict that took place 3,000 years ago between the Demon Clan and the 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 every time he died without fail, while the Supreme Deity cursed Elizabeth by making her reincarnate as a human every time she dies, 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 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.