Veritas (species)
The Veritas were an ancient alien race with dwindling numbers. The survivors of the race took extremist views on the truth, killing anyone who had lied, due to the fact that the three surviving Veritas had been tricked into buying alien technology which they believed was a power source that could ultimately make their homeworld more powerful. The tech actually contained a virus which destroyed all life on the planet, leaving only three Veritas - the three which had brought the technology back to the world - alive since they were inside a temple which protected them from the virus.
The Veritas realized that their whole race had been destroyed by a single lie, so they began hunting down people and aliens across the universe who had been known to have lied. They carried this on for centuries and eventually found out about Sarah Jane Smith, a woman who had kept the secret of extraterrestrial life in the universe a secret to the rest of the world, and became hell-bent on punishing her.
However, when the Veritas attempted to punish Sarah they discovered the pure chaos that their presence on Earth caused and decided that Sarah Jane was in fact doing the right thing by keeping what she knew a secret as some humans couldn't handle the truth of knowing about alien life. After their realization the Veritas let Sarah go. (AUDIO: Judgement Day)
Some time after this event, the Veritas became interested in Earth again. They created simulations or, practice Earths, in which they could practice world domination. The Doctor, Bill and Nardole were all unknowingly part of the simulation and, after a call from the Pope, the Doctor began working on a mystery in the hidden Library in the Vatican.
A book, which shared the same name as the Veritas, had been discovered. Nobody could read the book's few pages without succumbing to taking their own life and one of the translators had finally decoded the book and uploaded it to the internet. Soon, the Doctor discovered that the book revealed the truth - that the world they all inhabited was not real and was just part of the Veritas' plan - and that the people killing themselves were actually "switching themselves off" to ensure that they couldn't be used at the Veritas' will. (TV: Extremis
