Metagenic devices were powerful biological weapons outlawed by most major governments in the 24th century. A metagenic device was a genetically-engineered, airborne virus that destroyed all DNA it encountered, and was capable of destroying entire planetary ecosystems within days.
A force armed with such a device could introduce it into a target planet's atmosphere to rapidly eliminate all life – and resistance – while leaving all infrastructure intact, and within a month, the metagenic virus itself would become inert, essentially leaving the planet conquered.
In 2369, Starfleet Intelligence suspected the Cardassians of developing a method to launch dormant metagenic material on a subspace theta-band carrier wave in a secret base under the surface of Celtris III. This was actually a ruse to lure Captain Picard out and capture him, as he was Starfleet's only active member with knowledge of theta-band devices. (TNG: "Chain of Command, Part I", "Chain of Command, Part II")