The Veritans are a very peculiar species, which look like small elongated creatures with a pair of prongs which can cling onto a humanoid's face. They are almost completely inert and totally mute when on their own, but once clinging to their hosts they start speaking aloud any kind of statement as long as it's true, or widely perceived as true. They apparently do it involuntarily, but the damage it causes can be enormous as all sorts of unpleasant truths tend to come out.
As it turns out, the Veritans are actually a sapient species enslaved by the Dipthodat, which feed off the negative energy generated by the resulting conflicts after selling their "Truth-Tellers" as the latest fashion on the planet they are currently exploiting. The Veritans themselves likely feed off some sort of psychic energy as well, since it is mentioned that they never require food or cleaning up.
- Tip of the Tongue, by Patrick Ness (2013)