Board Thread:Questions and Answers/@comment-100.33.47.60-20170703220718/@comment-2605:E000:160F:19CF:51BB:7087:2BA6:4D90-20191123225133

Essentially long story short, Meliodas betrayed the demon race after meeting the original goddess elizabeth. At the time, Meliodas because of his power was a match for the goddess races 4 archangels. When he leaves the demon race this causes a shift in the balance of power and leads to the holy war, which is why in the anime the 10 commandments say it's his and Elizabeth's faults the holy war happened. I dont wanna spoil too much theres alot involved in the holy war but essentially the curse is a punishment put on them by the demon king. At first its believed that it's just a punishment but the demon king really did this for a reason, to revive himself by feeding on meliodas' emotions for liz each time he dies, and to corrupt meliodas to force him to revert back to the way he was before meeting elizabeth.