Centaurian slug

A Centaurian slug was a small parasitic animal which entered a humanoid host through the mouth, eventually traveling through the body to latch onto the brain stem, where it released a toxin forcing the host to answer any and all questions posed.

In 2258, the Romulan Nero used a Centaurian slug in his interrogation of Captain in a successful attempt to learn the subspace frequencies of Starfleet's border protection grids surrounding Earth.