A pathogen carried in the bloodstream of the future Admiral Janeway, who allowed herself to be assimilated so that it would infect the Borg Queen and wreck the Collective's command structure.
The neurolytic pathogen is the hidden weapon of the Voyager finale Endgame. Admiral Kathryn Janeway travels back from the early twenty-fifth century to bring her younger self's ship home through a Borg transwarp hub, and before her last journey she injects herself with the agent.
While Voyager attacks the hub, the Admiral goes to the Unicomplex and lets the Borg Queen assimilate her. The pathogen passes into the Queen and from her into the hive mind, cutting her links with her drones and causing her own body and the surrounding structure to break apart.
The Admiral dies in the destruction, but the confusion she causes allows Voyager to destroy the hub from within and reach Earth.