WARNING: The following contains spoilers for Season 5 of The Seven Deadly Sins, now streaming on Netflix.
The Ten Commandments have been the primary antagonists for the majority of The Seven Deadly Sins. The Demon Clan's most elite warriors, each possesses an absolute power known as a Commandment. Created by the Demon King himself, the race to gather the Commandments is the key storyline for the first half of Season 5. But what the Commandments are and how they work might not be immediately obvious.
Over 3,000 years ago, the Demon King's first attempt at creating a powerful servant failed when the Original Demon staged a rebellion against him. After punishing the Original Demon for its actions, the Demon King took a different approach. Splitting his power in half, keeping one half for himself, he divided the other 50-percent into 10 powerful magical curses known as The Ten Commandments. Handpicking his ten most elite warriors, he bestowed one Commandment upon each of them.
Each Commandment is, other than a source of incredible power, a curse that punishes those who violate its statute. For example, Melascula possesses the Commandment of Faith. Anyone, even Melascula herself, who displays a lack of faith in her presence will have their eyes burnt out of their head. Each Commandment's curse is a method of ensuring that none of their wielders ever rebel against the Demon King.
The power of the Commandments is absolute. Even Ban couldn't attack Estarossa, who wields the Commandment of Love, because its curse prevents anyone who holds hatred for Estarossa from being able to harm him. The only way to be immune from a Commandment's curse is by possessing one of the other Commandments. This means that Gloxinia, the Commandment of Repose, would be able to kill Grayroad, the Commandment of Pacifism, without Grayroad's curse causing Gloxinia to rapidly age and die. Likewise, Grayroad could fight Gloxinia without losing his magic, the curse of Gloxinia's Commandment.
This immunity from each others' Commandments is what keeps the balance of power amongst The Ten Commandments in check. Once a Commandment's curse has been inflicted, the only way to remove it is by the death of the wielder. So, anyone who turns their back to Zeldris, the Commandment of Piety, would become his slave for as long as Zeldris lives. Commandments like Patience and Selflessness, which curse violators with unceasing pain and a loss of identity, respectively, become agonizing torture with their permanence.
As gifts from the Demon King, Commandments are not permanently tied to their wielder. A Commandment can be removed from its user either with their consent or after their death. Additionally, inheriting a Commandment immediately turns a member of any race into a Demon, though they keep the characteristics of their original race. A member of any race is capable of wielding a Commandment, as Giants, Fairies, Goddesses and even a snake have been in possession of a Commandment at various points throughout the series.
While it is possible to acquire multiple Commandments, it's not exactly a great idea. Only Meliodas and Zeldris, as true sons of the Demon King, are capable of wielding multiple Commandments without losing control or just outright dying. Once Estarossa, later revealed to actually be the Archangel Mael, collected the Commandments of Love, Purity, Reticence and Truth, he quickly lost his mind. However, his attempted power grab did reveal that the Commandments are sentient to some degree, but they need a host to function properly.
In order to break the curse on Elizabeth, Meliodas decided he would obtain all ten Commandments, believing he would obtain power equal to that of the Demon King, the one who cursed Elizabeth in the first place. His logic appears sound, as the Commandments together represent half of the Demon King's power, while he still possesses the other half, so they would technically be equal in strength.
Unfortunately for Meliodas, the Commandments have further abilities, unbeknownst to anyone in The Seven Deadly Sins. The Demon King can spy on anyone who possesses a Commandment, reminding his son Zeldris that the Commandments are the Demon King's power, after all. Worse, once all 10 Commandments inhabit a single person, a feat only Meliodas or Zeldris could accomplish, that person immediately becomes the Demon King's vessel. One last failsafe against rebellion, the Demon King reveals that he has been using the Commandments to play chess while his sons played checkers.