The Pax, or to give it its full name, G-23 Paxilon Hydrochlorate, was a sedative used in a social engineering project that failed disastrously. The Union of Allied Planets released it into the atmosphere of a newly settled world named Miranda in the year 2506.
In very short order, the Pax pacified the vast majority of the population there into complete inactivity. They simply stopped, wherever they were and whatever they were doing, and waited, inactive, until they died (presumably from starvation and dehydration, in most cases).
They were the lucky ones.
One tenth of one percent of the population found that the drug instead boosted their aggression to a level described as ‘beyond madness’. They became brutal, sadistic and cannibalistic, given to acts of rape, torture and self-mutilation, and were known as Reavers. The truth of their origin was revealed to the galaxy at large by Malcolm Reynolds and his crew in 2518.