Negavolt Cultists are Heretek Cults that are often found on worlds controlled by the Dark Mechanicum. They fuse themselves with corrupt machinery that channels their own pain and hatred into arcs of Warp-infused energy that leaps from their Electro Gonads. Negavolt Cultists also express their heretical devotion by despoiling machinery and corrupting the function of sacred technologies through daemonic invocations and abstruse rituals.
Due to their nature and relative importance in the maintenance and construction of heretical machines and Daemon Engines. Negavolt Cultists can be found in both Chaos Space Marine armies and the Lost and the Damned. Although in the case of the Chaos Space Marines, they are often overshadowed by either a Warpsmith or a Tech-Assassin.
They made their debut in Blackstone Fortress under the Chaos Warband called the Servants of the Abyss.
The short story "Negavolt" gives more info on the cultists' signature weapon, the Empathic Resonance Coil, revealing that the cursed machines are infected with shards of a malicious (but nameless) data-daemon. The data-daemon compels the machine to infest the cultist's body, and once irrevocably merged physically/psychologically tortures them (sometimes accidentally to death) to gorge on their suffering and power its weaponry. Grimdark.
In tabletop 40k, Negavolt Cultists are literally just a unit of four Electro Priests with a 6+ save in addition to the 5+ FNP. In an already melee focused Chaos meta, they are a hard pass. It might be a good idea to mount these fuckers on 25mm bases and call them Heretek Marauders, since their walking poses would make that easier than the regular Electro Priest's standing.