A cloaked space mine able to build its own replacement, laid in a field across the mouth of the Bajoran wormhole in 2373 to keep Dominion reinforcements out of the Alpha Quadrant.
Self-replicating mines are devised in the Deep Space Nine episode Call to Arms. With Dominion convoys pouring through the wormhole, Sisko decides to mine the entrance. Rom has the idea of making each mine small, cloaked and fitted with a replicator, so that when one is destroyed its neighbours replace it.
Dax, O'Brien and Rom lay the field from the USS Defiant under fire, and its completion provokes the Dominion attack that captures the station and begins the war.
During the occupation, Gul Dukat's officer Damar finds a way to disable the mines with an antigraviton beam from the station's deflector array. In Sacrifice of Angels, Rom fails to stop the beam in time and the field is destroyed, but the Prophets make the waiting Dominion fleet vanish inside the wormhole.