The Veritas were an ancient alien race. The survivors have taken an extremist view on the truth, killing anyone who lies.
Three Veritas bought a power source from another alien race. They were told that this power source could make their home planet more powerful. However, it turned out to be a virus which destroyed all life on the planet. Only the three Veritas who bought the virus survived, since they were inside a temple which protected them from the virus. The Veritas realised that their whole race has been destroyed by a single lie, so they started to hunt down people and aliens who lied.
They did this for centuries until they found out about Sarah Jane Smith. Sarah Jane Smith had lied a lot to keep the existence of aliens hidden from humans and the Veritas wanted to punish her for that. However, when they saw the chaos that was caused when some humans found out about aliens they realised that Sarah Jane was right about hiding the truth. They let Sarah Jane go. (AUDIO: Judgement Day)