Pain Weapons
Pain weapons is pretty obvious from its namesake. They are weapons designed to make their victims scream in agony as they slowly die a horrible, bloody death. Just for the kicks. Naturally, only the Dark Eldar use these cause even Chaos have some standards.
Usually most Dark Eldar weapons are sadistic in nature and enjoys flicking as much pain as possible. But Pain weapons take one step further by using pain as the only form of death. Basically, they die from pure shock.
Usually, the most common users are the Pain Engines of both varieties.
Dark Eldar
Dark Eldar Combat Weapons
Lets be frank. The majority of DEldar close combat weapons are a subtype of Pain Weapons.
See main page, Dark Eldar Combat Weapons
Stinger Pod
One of the primary and signature weapons of the Talos.
Talos Pain Engines are often armed with sinister weapons that fire great pulses of raw agony. You hear that right, these guns kill their victims with the literal power of PAIN. Don't ask us how that actually works (most likely by massively over-stimulating the victim's nervous system, causing the brain to shut down and the victim to asphyxiate, suffer cardiac arrest and, well, die). Unlike the Ossefactor, which is a weapon that relies on the body modification and sheer pain shock to kill its targets methodically. The Stinger Pod as aforementioned, uses the very concept of pain as a weapon.
The victims of these weapons often break their own bones or rupture their own organs with the force of their agonized convulsions, while survivors are left catatonic with pain, unable to defend themselves against the Talos' bloody ministrations.
Spirit Vortex
A strange contraption found only on the backs of a Cronos Parasite Engine.
Cronos Parasite Engines are sometimes modified to incorporate a spiral-etched device that can hurl out a massive burst of negative force, draining the lifeforce of dozens of victims. The range of which is not yet accounted for.
Spirit Vortexes are just one of the primary tools needed for the Cronos to soup up the condensed agonies of the victims they have sucked the life out of. These vampiric practices are needed to also fuel the Stinger Pods of its brother engines.
Most often then not, the victims of the Spirit Vortex are subjected to suffer intense agony for it to be possible in the first place to harvest these condensed agonies. It is one of the strangest Pain Weapons available to the Dark Eldar.
The Cronos' other weapons such as the Spirit Probe uses the same practice only in close combat whilst the Spirit Syphon is a field of baleful energy that has been cast out from the Cronos itself. Usually the affects are all the same. The emaciated corpses of a victim in its last grasp to scream.
Chaos
Tormentor Cannon
The only Chaos Pain Weapon to date.
The Tormentor Cannon is a large, long range Warp energy weapon that is used by the Slaanesh Subjugator and Questor Scout Titan Daemon Engines as as the creature's primary ranged weapon. The Slaanesh Subjugator is a large Daemon Engine that is completely dedicated to the service of Slaanesh, the Chaos God of pleasure and excess.
As befitting the Dark Prince of Pain and Pleasure, the Tormentor Cannon is a psychically-charged Warp energy cannon that kills the Subjugator's victims with unbearable agony, leaving a trail of charred wrecks and ashen corpses wherever it is fired. The Tormentor Cannon is used in pairs by the Slaanesh Subjugator, one mounted atop the machine's scorpion-like tail, and the other mounted dorsally along the daemonic machine's back. It is unknown if the Tormentor Cannon is used by any other Chaos war machines or Daemon Engines.
Tl;dr, this gun causes so much pain to its enemies that they literally burst into flames from all the screaming.