The Centaurian slug is a small parasitic creature used as a means of torture and interrogation. It enters the host's body through the mouth, then attaches itself to the brain stem. Once attached, it secretes a toxin that functions as a truth serum, preventing the host from being able to withhold any information when asked. (TOS movie & novelization: Star Trek) The centaurian slug's anatomy and lifestyle bears similarity to the Ceti eel.
In 2258 of the Kelvin timeline, the Romulan criminal Nero had Admiral Pike in captivity. Nero inserted the slug into Pike in an attempt to glean the frequency of Earth's border protection grids from the admiral. Due to the influence of the slug, the interrogation proved successful. (TOS movie & novelization: Star Trek)