The Council of Guardians, also known as They Who Wait, was a lost tribe of Bajorans that lived underground on their homeworld where they lived a shadow existence to the rest of their civilization. They did not recognise any political body and considered themselves a religious faction who were given their mandate by their Gods. This shadow society was known for keeping strict control over their population numbers in order to maintain their existence.
According to legend, They Who Wait was formed as a result of an aspect of Bajoran mythology involving the Battle of the Gods and their herald known as The Messenger. Ancient beliefs stated that every two thousand years, a terrifying conflict erupted amongst the Gods which devastated the planet Bajor and wiped out the surface civilization. However, after a generation of living underground, the Council of Guardians would then send some members of their tribe to the surface in order to rebuild their civilization. Both scientific and archeaological studies revealed that the source of this disaster was a comet which the Bajorans knew as the Messenger and it's entry into the Bajoran wormhole led to a spread of high level radiation across several hundred light years which was responsible for the purges of Bajor's surface. Thus, their religious beliefs involved a death and rebirth cycle which they were fanatical in their devotion to maintaining this process.
Following this planetary disaster, They Who Wait continued their underground life whilst legends of their existence was spread amongst the newly forming surface people who were known as Uplanders. By the 24th century, the Council of Guardians began to make preparations for the coming of the Messenger whereupon they would repopulate the planet. When Commander Benjamin Sisko considered using Deep Space 9 to destroy the comet, the Council of Guardians decided to reveal themselves and their leader journeyed to the space station. There, they first demanded that Sisko did not interfere and later turned violent as they intended to ensure the arrival of the Messenger by crashing a transport into the space station as well as boarding it in order to stop any attempt at interfering with the comet's path. However, their attack failed and Lieutenant Jadzia Dax was successful in diverting the comet and preventing the destructive cycle of death and rebirth. After this event, the Council of Guardians were arrested and the Bajoran government denied any involvement in their actions. (DS9 comics: "Dax's Comet, Part 1", "Dax's Comet, Part II")