A weapon devised aboard Voyager that delivers modified Borg nanoprobes in torpedo casings, the only armament able to destroy the bioships of Species 8472.
Species 8472 could not be assimilated because their immune systems destroyed anything that entered their cells. In the Voyager two-parter Scorpion, the Doctor finds a way to reprogramme Borg nanoprobes so that they slip past those defences, and Janeway offers the technique to the Collective in exchange for safe passage through Borg space.
Working with a group of drones led by Seven of Nine, the crew load the altered nanoprobes into photon torpedo casings. The Borg want a single enormous warhead that would wipe out everything across light years; Janeway insists on smaller weapons as a demonstration.
The warheads are used inside fluidic space and again on return, shattering several bioships and persuading Species 8472 to withdraw from the galaxy.