The Messenger was a comet that was known to engage in a two thousand year cycle where it entered in the Bajor system and intersected with the Bajoran wormhole thus generating deadly radiation across hundreds of lightyears that wiped out entire civilizations. Scientific analysis of the comet revealed that it had a hydrogen core larger than a small sun with the interior possessing elements of a heavy hydrogen radioactive isotope that was known to exist only in a laboratory and resembled titrium. Thus, when the unstable elements interacted with a wormhole, it created an atomic fission reaction that created an apocalyptic event which destroyed everything in a star system.
This comet featured heavily in Bajoran mythology and legend stated that it was responsible for a purge of Bajor's surface around two thousand years ago thus wiping out the surface civilization. Archaeological evidence supported this suspected event as traces of such radiation was detected on Bajor's surface. Whilst it had wiped out the surface population of Bajor in the past, a hidden shadow society living underground known as the Council of Guardians continued their existence for a century whereupon they sent some of their number to rebuild the surface world. As a result of the comet's actions, a belief system was arranged around it and it was suspected as being the involvement of the Bajoran Gods with it arriving during the momentous Battle of the Gods which the believers felt was responsible for the cleansing of Bajor. The Council of Guardians, who were known to the rest of the Bajorans as a lost tribe called They Who Wait, believed themselves to be instruments of their gods actions and that the Messengers involvement was critical in their destiny in the cycle of death and rebirth.
In the mid 24th century, the Messenger was once more on a path to the Bajoran wormhole and Council of Guardians awaited for its deadly purpose to be manifested. At this point, hundreds of Bajorans attempted to depart their home system as they believed in the oncoming destruction of their world. However, Commander Benjamin Sisko attempted to take action in stopping the comet which prompted the emergence of They Who Wait. Their leader along with his entourage arrived at the station where they caused a transport to collide with Deep Space 9 as well as engaged in a large boarding action in order to stop any attempt at circumventing the Messenger's movements. Despite this being the case, the crew of the station were successful in using Runabouts to alter the trajectory of the comet based on calculations by Lieutenant Jadzia Dax. Thus, the crisis was averted and the Council of Guardians members were arrested for their actions. Whilst the disaster was prevented, Dax felt a personal guilt as she felt that the most minor of miscalculations might mean that the Messenger's course was forever altered and it might be now responsible for the destruction of a number of civilizations on its new path. (DS9 comics: "Dax's Comet, Part 1", "Dax's Comet, Part II")