"Even a man who has nothing can still offer his life."
- – Thought for the day
Space Marines are big tough supermen and all, but even they need a helping hand. Someone has to to polish their armor and nob, cook their food, sweep their halls, clean their toilets, file their tax returns, and keep the fortress-monastery defended when the brothers are out stomping Bloodletter faces into the ground and tearing Ork rectums apart with chainswords. After all, Space Marines are knights in spaaaace! and every knight needs his retainer. These people are Chapter Serfs and live comfortable, well-to-do lives... at least by Imperium of Man standards.
Chapter Serfs (formerly Legion Serfs), were originally picked from failed Space Marine Aspirants and other natives of Space Marine homeworlds who, though physically unable to become Space Marines, still wished (and were deemed able and worthy) to serve. The children of Chapter Serfs also often become chapter Serfs themselves. Generally speaking, Chapter Serfs are well-trained, highly educated, and treated well; they are officially part of the Chapter, and therefore get access to the fancy toys in the space marine armoury.
How well they're treated depends on the Chapter they serve. Ultramarines, Blood Angels, Space Wolves, and Salamanders are well aware they couldn't purge aliens, mutants and heretics without the 'unglorious' support Serfs provide and treat them as equals to other Space Marines, like soldiers might treat their friends in the supply corps. The Wolves use their serfs as a defence force on Fenris, known as "Kaerls", though we don't know if they get to go on campaign or not except for those serving in the fleet. The Star Phantoms entrust them with maintaining the chapter's faith and up keeping their massive ossuary. Blood Drinkers, being vampires, let them volunteer to be ritually drained of blood for them to (surprise surprise) drink to stave off the Red Thirst. Iron Hands and Imperial Fists and their countless successor chapters see them as just kind of there. They don't really care one way or another and form little bonds with them, considering service to be its own reward (think the way you treat the fast food workers and garbage people who make your comfy life possible). Marines Malevolent, being dicks, treat them like slaves. Flesh Tearers, being psychotic, treat them like stress balls to work out frustration on between missions. (Spoiler alert: a human can't usually survive being used as a stress-ball by a Marine.)
As for the the Traitor Legions, it varies from "tolerably bad" to "absolute hell", mostly depending on the guy in charge and what God(s) he worships. Most often though, Chaos Warbands doesn't have actual Serfs anymore but use Cultists who are pulled towards the Warband to worship them as living avatars of the Chaos gods. Cultists are most of the time treated like crap by the Traitors, since their own hubris and newfound freedom doesn't go well with 'being nice to people below you'. But on the other hand, Chaos Lords knows how important a healthy supply of fanatic cults is for the runnings of a Warband and the value of meatshields in combat. Even bands of Khornate Berzerkers still have mortals tagging along with them despite their constant urge to kill anything crossing their path in the name of the Blood God, which means they do show some restraint. (of course, said mortals are just as bloodthirsty and just as often end up killing some of their own, but Khorne cares not from where the blood flows).
Of the various Traitor Legions depictions so far, the Emperor's Children are the worst, mistreating their "serfs" to death so hard they had to resort to stealing everyone else's during the Great Scouring (causing a huge civil war amongst the Traitor Legions). The Word Bearers mostly see mortals as a reserve of walking sacrifices waiting to happen. One band of Iron Warriors used slaves instead of chapter serfs, seeing them as nothing but replaceable cogs in their machines of war (and they ended up being just as reliable as you'd expect when the shit hit the fan); Honsou however sees the armies of cultists in his service as low-quality troops that nevertheless have their use so he values them a tiny little bit. The Thousand Sons use a combination of Tzaangors (who are actually voluntary employees who are treated relatively well and respected) and enslaved native human population from the Sortiarius (who, should they manage to escape and survive, become primary candidates for new Thousand Son marines. That's how Tzeentch rolls). The Crimson Slaughter has an entire city's worth of serfs in the depths of their battle barge that they just ignore most of the time (they tag alongside the Crimson Slaughter in battle hoping to get noticed, and those that somehow distinguish themselves get a shot at becoming new Crimson Slaughter marines). Talos Valcoran of the Night Lords treats his serfs surprisingly well (better than even some Loyalist Marines), and the others in his warband at worst see mortals as useful tools that shouldn't be wasted pointlessly.
Alpha Legion warbands operate at the halfway point, maintaining unaugmented operatives who, depending on the mission, stand a roughly fifty-fifty chance of being trusted with roles vastly more important than the Astartes themselves or being sacrificed to the enemy in interference action. Like the loyalists, these are considered members of the chapter. Like most other traitors, that makes then far from indispensable.
The Blood Gorgons have the most unique relationship with their serf's amongst the Chaos Space Marines, not only treating them with respect but even mortals in general. While this might sound a bit sueish it's actually a essential part of their war against the imperium, acting as military supporters for off world rebellions, setting up recruiting worlds much like their loyalist brothers and allowing the chapter to control quite a number of planets despite being a much smaller weaker chapter then their competition. It's implied that other Chaos Marine chapters give them shit for this, though it's unlikely they'll be changing their ways any time soon thanks to how this fair treatment came back around in their darkest hour.
See, when the Blood Gorgons were betrayed from the inside allowing their space hulk to be captured by Death Guard, not only did did the population of their recruitment worlds fight back (as in, took on Nurgle Marines with bows and arrows) but while the chapter serfs were imprisoned aboard their hulk they threw themselves at their marine captures and literally suffocated them under their mass of bodies, causing utter chaos all over the ship and allowing the Gorgons to break out and fight back. For the record the actual treatment of Blood Gorgon serfs involves acting like they were nine to five employees, letting them just do their dam job before having free time to chill (and take drugs. LOTS of drugs.)
In summary they are the Alfred of 40k, because no fine super hero are without a friend/allies that would comfort them after all the grimdark shit happened outside to fight another day.
In 'Spear of the Emperor' by ADB, the Mentors have a specialized subset of Chapter Serfs called "Helots" that are tasked with remotely controlling laser shooting Servo-skulls to give their Masters targeting information and suppressing fire (like those drone pilots in real life's M3 era), maintaining the Marines' gear, and fighting back against attackers in the case of their ship or base being attacked. While armed with power blades, armor piercing shotguns with vortex grenade launchers, your basic assortment of laspistols & throwing daggers, and Carapace Armor that's advanced enough to put Stormtrooper equipment to shame, they're still treated as peripheral (albeit useful) servants at best that are below their Masters' immediate concerns.
Granted, even serfs are specialized in specific roles and not an all-rounder 'slave'. Here are the most common roles that serfs might serve in a Chapter
- Artificers - Technicians and assistants to Techmarines. Routinely polishes and conducts minor repairs on their master's power armor and wargear while the Techmarines handle the more complicated tasks.
- Master of the Refectorium - These are the 'head chef' of a Chapter's kitchen, ensuring the hefty amount of food served is prepared at its highest quality. Some of the most prestigious position that a serf might get into since even Space Marines would love to have a well-served meal.
- Medicae - Nurses to the Apothecarys' doctor. As the Space Marines' anatomy is complex beyond comprehension, these are often staffed by the most intelligent and dexterous serfs.
- Monitors - Messengers, engineered to be able to remember pages long worth of information and SOMEHOW ENCRYPT THE TEXT and DECRYPT it when given correct verbal code. Fleshy 1940s Enigma Machines, creepy
- Sacratium - Lay assistants to Chapter's Chaplains. Tends Chapter's relics and assists in perform ceremonious tasks. In chapters with strong religious leanings, these serfs can even outrank marines.
- Scribellum - Clerks to the Chapter's Librarians. Chronicle the Chapter's history and deeds, also assists in creating Purity Seals.
- Shipmaster - The commanding officer of a ship in the Chapter's fleet -- they would be the Captain if that wasn't their boss' rank. They oversee the ship's operations, including combat when the Space Marines are engaged in fighting off opponents.
Master of the Household
Comes under a variety of different titles depending on the Chapter, like "Lord of the Household" or "Ordinator of the Household" etc etc. They act as the Chapter Master's personal teddy bear and equerry. Well performed serfs (usually on the field of battle) can catch the attention of their Chapter Master lords and get promoted to this position. Acting as the nominal head of the Serfs overseeing the mundane duties of food preparation, hygiene, maintenance and construction tasks as well as overseeing serf recruitment (in situations where selecting from failed Astartes candidates is not enough). These folks ensure that other Serfs are smoothly performing their duties and get the special privilege of being in the presence of the Chapter Master at any time.
Unusually for a position within the Chapter, this post can be filled by either a Serf or a full fledged Space Marine. Back in the old days this position was said to be difficult, but mostly boring, considering that it's not a frontline position. Yet if a Marine took the position it was seen as allowing them to fulfill the necessary training that would enable them to progress to higher rank within the Chapter, for instance: 1st edition Marneus Calgar was Master of the Household before earning his promotion to Chapter Master. While this is less the case these days, Space Marines may still fulfill the role in different chapters, for example: the Blood Angels and Dark Angels both have a brother titled Warden of the Gates in charge of their Chapter Serfs.
Rules for Serfs
Chapter Serfs are represented with various rules both in the 40kRPGs and on the tabletop:
On the tabletop (both in 40k by the Badab War books, and in 30k Extermination) they are represented in battles where space marine capital ships get invaded by enemy forces, where the crew has to get involved by necessity. They come in squads of 6, are armed with either laspistol and melee weapon or shotguns, and wear void-hardened flak armour. However, they have the same stat-lines as Imperial Guard Conscripts, because they aren't battle-hardened fighters, or expected to be.
The Deathwatch (RPG) does it very similarly, having stat lines weaker than starting Dark Heresy player characters, and if you take the tens digit of their statline it very nearly exactly follows the tabletop stats for them, too. What is different about them, though, is that chapter serfs do have biological enhancements, and have the Unnatural Strength, Toughness and Senses traits, as well as Dark Sight, which actually falls into line with how the main character of Prospero Burns is presented (though he's not strictly a Serf, his enhancements were apparently no great expense for the Legion possibly because instead of giving him a augmetic eye they just rammed a wolf eye in there, but they replaced every organ system but his nerves with new ones).
However, with the ally rules, you could probably take them as Imperial Guardsmen, just modeled differently. What to do with all those Spare Marine and scout torsos? Give'em lasguns and play them as Chapter Serfs.