- "If they take the ship, they'll rape us to death, eat our flesh, and sew our skins into their clothing. And if we're very, very lucky, they'll do it in that order."
- -- Zoe, "Serenity"
Reavers are extraordinarily anti-social interplanetary raiders from the Firefly TV series.
Reavers are violently sociopathic toward all other beings. This condition was induced by exposure to a chemical ("G-23 Paxilon hydrochlorate") introduced into the environment of the Miranda colony, which was supposed to make the population calm and docile. The drug caused 99.99% of the population to become so docile that they essentially just let themselves die of starvation, lacking the initiative to even get up for food. A tiny portion of the population had the extreme opposite reaction to the drug, becoming violent sociopaths: the Reavers.
Reavers are human, but they are afflicted with an insane desire to capture and torture non-Reavers. They are also prone to self-mutilation and scarrification, making them quite frightening to behold. While they can be killed by ordinary weapons, they are highly resistant to pain, giving them zombie-like determination in combat. Reavers prefer to capture victims instead of killing them, so many of their weapons are non-lethal, such as dart guns loaded with paralyzing poisons.
It's not clear why Reavers don't turn on each other, but they evidently have some kind of social structure that allows them to cooperate to maintain a limited technology base and raid other colonies. It is not clear whether Reavers reproduce, although it seems unlikely that such a society could successfully raise children. At least one survivor of a Reaver raid was "contaminated" with their insanity by torture, but there is no evidence that this was anything but a unique, isolated incident.
The Reavers have cannibalized and modified a wide assortment of spacecraft from the Miranda colony to build a fleet of pirate ships. These ships generally operate independently, but they will mobilize in force to attack any intruder into their territory. Their ship-to-ship combat tactics are reckless and uncoordinated, making them dangerous primarily to poorly armed and disorganized civilian targets.
The Miranda colony had a population of approximately 30 million, of whom approximately 0.01% became Reavers due to the "Pax" exposure. Therefore, there are presumably about 300,000 Reavers, less whatever casualties they have incurred since their creation.
The Reavers have no FTL propulsion capability, so they present no significant threat to other star systems.