Stigma

was an alliance made between the Goddess Clan, Giant Clan and Fairy Clan, with humans joining later, to fight against the Demon Clan 3,000 years ago during the first Holy War. The headquarters for Stigma was located at the Fairy King's Forest, marked by the Light of Grace. 3,000 years after the Demon Clan's seal is broken, the Four Archangels (without Mael) find human vessels to inhabit and reform Stigma with the Giants, Fairies, the kingdom of Liones for the humans, and the Seven Deadly Sins (without Meliodas and Ban) to fight against the Demons once again.

Background
At some point during the Holy War, the Goddess Clan along with the Giant Clan and Fairy Clan would form an alliance against the Demon Clan, this would lead the Holy War down a worse path, later, Meliodas would betray the Demon clan uniting to Stigma.

During the war, the humans united to defeat the Demon Clan, but in fact they looked for revenge against Stigma for destroying their homes. Their rebellion ends with the death of Rou at the hands of an enraged Gloxinia.

Time after, the Demon Clan would finally be defeated and sealed by the Goddess Clan ending the Stigma alliance.

At present 3,000 years later, again in the war against the Demon Clan, Stigma was reformed by Ludociel while Sariel and Tarmiel personally inviting the relieved Holy Knights into the alliance after saving them from the Demon Clan's forces along with the Seven Deadly Sins and Elizabeth Liones joining the ranks to defeat Meliodas and the Demon Clan despite the risks.

To fight the Demon Clan, Stigma and the allied forces are divided in three groups. The "Defensive Force" designed to protect Liones from the demon's forces, the "Search-and-Destroy Force" designed to advance towards Camelot and confront and annihilate the army of the Demon Clan, and the "Assault Force" designed to launch a direct attack on Camelot.

Trivia

 * The word Stigma means a certain disgrace associated with a particular group or thing. This likely refers to the group's indiscriminate killing of Demons and all those who seemed to have aided them, such as the group massacring a human village that did no wrong apart from taking in and caring for an injured Demon.
 * Ironically, the original Elizabeth was an official member of the Stigma and her 107th reincarnation is a member through alliance.
 * Meliodas, who was once a member 3,000 years ago, became the main target 3,000 years later.