The commonplace military skeletrons have a well known and well respected place in the Atlantic Federation Armed Forces, and likewise in the arsenals of many other global powers. Likewise, there are many other types of common robots in service around the world, such as the frequently outdated autons, service droids, or the police model wojeks frequently used in less savory environs. These machines all share a common flaw, at their core is at least one lump of arcanotechnology, if not more than one. The inescapable factor is the neo-cortex that controls each of these robots, while many of them have arcanotech sourced power supplies, or carry arcanotech equipment or weaponry.
This arcanotech can become corrupted, and this corruption can expand outwards from the failed piece of equipment and infest the chassis of the damaged machine with a sort of cadaverous cancerous flesh that functionally reanimates destroyed robots. This phenomenon also happens with larger machines like mecha, creating monsterous Hekatonkheires, and with stationary arcanotech devices, which typically result in Dimensional Fatigue Events, which are much larger in scale and severity.
The Necrotron retains the typical outside appearance of the make and model machine it was before it was damaged and became infested. If the outside of the carapace has been damaged, the flesh growing inside the machine can easily be seen, and machines that are severely damaged, missing limbs and such, will have functional replacements grown from their exoplasmic biomass. That being said, these limbs are as likely to be writhing masses of cilia, tentacles, clattering insectile legs, or worse things.
Necrotrons retain fragmentary records of their original functions, including self recognition, tool and special ability uses, and display an animal cunning that is sometimes more frightening for their mechanical origins. Military units retain the ability to pick up and use weapons, and can power arcanotech gear off of their own energy patterns, ignoring the need for a power supply. These weapons discharge with a resonance matching the necrotron firing it, thus a necrotron with a freeze cannon will functionally have a necrotizing freeze cannon.
The first necrotrons were created by accident, damaged and destroyed skeletrons left to rust in a dimensional fatigue zone became infested and rose to resume their original task, killing. This remains a common, but not the most common method of their creation.
We are regularly informed that the danger of the shadow cults is their ability to subvert civil society, spread mayhem and chaos, and otherwise disrupt our safe and peaceful lives. It's propaganda to keep the real threat of the real cults a secret. The real capital C Cults have access to parapsychic and arcanotech that falls into the category that most people would call magic. The Cults have the ability to create monsters deliberately, induce dimensional fatigue events, and cause Fortean events with frightening ease. But, that can't go on the evening news. There would be a massive revolt against the governments for not dealing with them, a surge in their membership and people seeking the sort of terrible things they have, and a falling out of the world's remaining few religions. Why pray to a silent god that doesn't answer prayers when there are things that live in the shadow of the world that will teach the arts of magic and resurrection?
There are a number of small cults of elite parapsychics and cosmically tainted individuals who have the ability to cause a machine to become a necrotron, inducing a sort of 'bloom' inside the machine that destroys the programming and arcanotech equipment. Whatever the robot had been before, afterwards it is a killing machine. These magically created Dead-Bots function under the thrall of the individual that created them, and can be much more dangerous than the free-formed ones.
Cult Born necrotrons retain the greater part of the skillsets while remaining functionally feral technozombies. They are also seldom as damaged as free formed individuals are. The Cult Born Necrotrons are frequently found in the vanguard of cult defenses, using their high durability, quick regeneration, and feral zeal for killing to great effect.
Deployment and Frequency:
The Necrotron is not a common foe, as the time and energy it takes to create them can be prohibitive. The counter side to this is that if one is encountered, the chances of running into more of them drastically increases. Skeletrons and other such machines are very common, and if the conditions were right for one to reanimate as a monster, there were probably plenty more who did the exact same thing. Likewise with the Cult-Born, if a group had the skill to create one, they have the skill to create as many as of them as they could find robots to use.
Combat and Greater Purpose:
The Necrotron is for all intents and purposes, an armored zombie. With an armored shell and metal bones, they are very tough, and the only way to destroy them completely is to either incinerate the corrupted components or totally destroy the arcanotech component that anchors the necrotron.