A genetically engineered virus that destroys every form of DNA on a planet and then breaks down, leaving the cities and industry intact for an invading force.
Metagenic weapons are described in the two-part episode Chain of Command. Released into a planet's atmosphere, the virus mutates to kill everything living within days. A month later it has become harmless and the attacker can simply walk in. The Federation and its neighbours have banned them.
Starfleet Intelligence picks up theta-band subspace emissions suggesting that the Cardassians are developing a safe way to deliver such a weapon. Picard, one of the few officers with experience of theta-band carrier waves, is sent with Worf and Doctor Crusher to destroy the laboratory on Celtris III.
There is no laboratory. The signals were bait intended to draw Picard in particular, and he is captured and handed to the interrogator Gul Madred.