Stigma

was an alliance made between the Goddess Clan, Giant Clan, and Fairy Clan to fight against the Demon Clan 3000 years ago during the old Holy War. The headquarters for Stigma was located at the Fairy King's Forest, marked by the Light of Grace. 3000 years later after the Archangels found human bodies to take over they decided to continue with the Stigma. They formed an alliance with the Seven Deadly Sins and Elizabeth Liones.

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 clan of the devils uniting to be able to defeat them.

During the war, the humans united to defeat the Demon Clan, but in fact they looked for revenge against Stigma for destroying their homes and finishing 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, 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

 * Ironically, Elizabeth (original incarnation) was an official member of the Stigma and her 107th reincarnation is a member through alliance.
 * Meliodas, who was once a member 3000 years ago, is now the main target 3000 years later.