What it's like
For the sake of argument, let's say you're Helikaon the Mourner, a Word Bearer Chaplain in the Heresy. You're part of the command team, with Captain Vilus (maybe he has a personal title like Archon, Consul, Warleader, or whatever else), essentially co-leading the Company, within the Chapter, within the Host of several Chapters gathered together. Or just one Chapter. Or just half of several Chapters banded together for a few months, years, decades, or centuries. Maybe all of your Chapter/Company remnants form a Host for convenience, or because Lorgar ordered it, or you're part of the same Crusade Fleet, or you just owe each other bonds of absolute brotherhood. But let's focus on you, Vilus, and the Chapter of the Sacred Phoenix Unbound, named after one of the constellations above Colchis.
Maybe on the tabletop, you're a Chaos Lord, or a Dark Apostle, or something the rules don't have a place for, because they're merely the basic skeleton structure. Let's move on to what matters.
Time passes in the Eye of Terror, after the Sons of Horus lost you the war by running when their primarch fell. You spend a lot of that time fighting the Sons of Horus because they're weak, and getting weaker. You kill them and take their ships, their worlds, their fortresses, their equipment. You make deals with worlds settled by the Dark Mechanicum. Other times, you invade those worlds, for supplies. Other times, the Mechanicum breaks its oaths to you, screwing you over, because that particular forge, that particular leader, or that particular world had a better offer from a stronger warband. Or because they secretly hated you. Or because of reasons you never find out.
You raid the Imperium a lot, for vengeance, for supplies, for slaves, for territory, and to teach them the truth behind reality. Maybe you do it for the glory of the Dark Gods, to please them, or maybe you do it for yourself, to make life better, easier, or to humiliate a rival who lost a fight against that world or fleet in times past. Maybe you do it because there's pressure from several Aspiring Champions and squad leaders that are inciting the troops into believing you're weak, and should be removed from your position as co-leader. You need a show of strength. Soldiers need victories.
You make alliances with other warbands from other Legions, either to group together against a serious enemy, or because you share territory and genuinely consider yourself allies. You break some of those promises, because it suits you better to kill those former allies and take their land/resources/ships. You stick to some of the others for all time, because they're your Gods-damned brothers in arms, and you'd die for each other. Only, in 6,000 years of living in Hell; fighting daemons; and fighting the Imperium, you betray them because you learn they're about to launch a surprise attack on your stronghold, or your fleet. Only, maybe they weren't, and you were deceived by a third party. Or maybe they were, and they win, and you have to cut and run with half your resources and manpower squandered in a war you never saw coming. Much of the time, no matter what happens, you fight a lot of other warbands from other Legions. That's life in the Eye of Terror. For every alliance you make, there are half a dozen battles.
Sometimes, perhaps often, you come into contact with other Word Bearer Companies/Chapters/Hosts persecuting their own wars. Sometimes you join together, share news of the Legion's movements, and are the best of blood brothers. Other times, it isn't so simple. These Word Bearers worship a different cult and creed to you, and your beliefs aren't exactly gelling smoothly. There are as many cults and paths of Chaos faith as there are preachers and worshipers, and just as real world religions and branches of the same religion come into conflict, it happens with Chaos faithful, on a much, much, much larger and more frequent scale. Maybe they force aspects of Tzeentch worship you think makes them weak (maybe they use lore of the Change God to read the hearts of their enemies, which relies too much on Chaos rather than being strong on your own), and they think the way you see nobility in Khorne makes you deluded. They think you take one aspect of your faith too far, or not far enough. You think the same of them. You both have evidence of why the other warband is weak, because no group is ever without flaw. Maybe a tense negotiation between your leaders on neutral ground becomes a gunfight. Maybe your fleets meeting by accident becomes an all-out void war. Maybe you reconcile your differences in the name of the Word, and become brothers for 3,000 years, either answering each other's calls for aid, or even joining into a new Host, of two Chapters bonded by absolute loyalty. Maybe you become a new Host of a whole new Chapter, to reflect your new unity. Maybe that alliance lasts for a year. Maybe it lasts until the end of time.
Other times, other Word Bearer warbands call to you for aid. Sometimes you answer, because you're the same Legion, damn it, and that matters. But sometimes you don't answer, because if that Host gets butchered, you can move in and claim their territory and resources much easier. Other times, you answer their call for help because you owe them; they've saved you in the past. Other times, you don't answer because those guys are oathbreakers and heretics, dangerously disloyal - a nasty splinter faction - and you want them dead anyway. Other times, you want to aid them, but you don't make it in time, because the Warp's tides delay you; or because astropathy is near-strangers interpreting each other's vague dreams, and you miss the message in the nightmare of a million screaming children vomiting black sludge from their mouths while they're skinned alive by chanting monsters with liquid flesh made of pus and filth and liquefied hate. Oh, that was a call for brotherhood by the Chapter of the Dark Maw? Not just one of the million nightmares you have, or the daemonic whisperings you hear all the time, because you live in Hell? Damn, Maybe you'll get it right next time.
Except maybe next time, the Host of the Dark Maw come and attack you for breaking an oath, and bring several other Word Bearer Hosts with them, who now despise you for breaking the loyalty of the Legion. They're loyal Word Bearers, but they see you as dangerously disloyal, a nasty splinter faction that needs to be destroyed.
Or maybe you interpret the message right, and are dying to come help your brothers, but the Dark Maw are fighting the Chapter of the Osseous Throne, and you owe both Chapters your allegiance, so the only honourable thing to do is sit the fight out. Or maybe you owe the Osseous Throne an actual oath of brotherhood from past campaigns, and can't take the Dark Maw's side. Maybe the Dark Maw are fighting a warband from another Legion - the Venomous Rune, of the Death Guard - who you've served with and allied with a dozen times. But you break your oath to your proven allies, because the Dark Maw are Word Bearers, and the Venomous Rune are not your Legion. It doesn't matter because Legions mean everything, and your loyalty is to your bloodline, knowing it will never fail you. Or maybe it does matter, this time. Maybe you take the field against a warband of your own Legion, because the alliances you've made in the years since the Heresy during the Legion Wars in the Eye of Terror are what matter most to you now.
At some point, Vilus is assassinated, leaving you in sole command of the Host. Now you're powerful, but vulnerable. Your own Aspiring Champions are, well, aspiring. They think they can lead the Legion better than you. They point to oaths you've broken to other warbands, or oaths you've made when many of your men wanted to break them; or battles that didn't go in your favour. It doesn't matter if their slander is fact or fiction, words spreads among the ranks. Some (many? most?) of your men harbour secret desires to replace you. Or maybe they don't, and you're just worrying over glances and stilled conversations and spy reports over nothing. You form an elite guard, but they take heavy casualties because they're in the front line of every battle. And can you trust them, really? They're in the best place to kill you if it came to an assassination. Maybe they're taking so many casualties becuse your other squads keep not deep striking in time. Is that intentional, or are the frequent repairs that need to be made really causing mechanical problems? Or is it that your ships are increasingly alive and sentient, half-daemon themselves, and harder to control with conventional means?
Maybe you focus on learning ancient , difficult-to-acquire lore on sorcery, to learn how to bind daemons more strongly and control your mutating fleet. Maybe you do it to control your own men. Only, the rebellion against you grows, because they say you're focusing too much on sorcery and not material conquering. Are you? You're sure you're not, but what choice do you have? This has to be done. You have to make things secure for the warband. Don't they see that? Maybe some do. Maybe others are still planning to kill you, and will have to learn in time that they'll go through the same doubts and plans when they're in your position.
A Crusade is called. Maybe one of Abaddon's Black Crusades; maybe one of the lesser Black Crusades called by another individual of great power; maybe just a massive raid into realspace. Awesome. During that campaign that lasts 24 years and covers battles on 30 worlds, you're attacked by a warband in an ambush, in the middle of a fight against the Imperial Guard, by an Emperor's Children warband you've never even heard of, and can't remember offending. Why? Are there reasons from the past? The Legion Wars? Maybe there are. Maybe they just saw a chance to screw a weaker enemy over, or force you to lose the fight so your Legion's reputation suffers. Maybe they were mercenaries hired by another warband to soften you up for a coming assault. Who hired them? You redouble your spy network, not knowing if you can trust them at all, since they said nothing about this ambush.
During the Crusade, you ally with a cabal of Thousand Son sorcerers, and their Rubric bodyguards. They have their own ships, resources, manpower - what a coup. They join your warband, though not really being sworn members of your Host, they'd still die for you and you'd die for them, with bonds forged in the crucible of war. They could've betrayed you a dozen times, but instead they risked everything to save you from one hell of a fight. You could've betrayed them and stolen their lore, but you saved them. Except maybe you didn't. maybe you need their sorcerous lore, so you slaughter them when they're weakened from helping you. No one will ever know. Right? Right? You get away with it cleanly, never hearing about the incident again, and enjoying their books of sorcery. The Thousand Sons never seek revenge. Except maybe they do, because they come 1,000 years later, in force, to annihilate you. Or maybe another Thousand Son warband thanks you for destroying their main rivals, and offers you a union. Maybe another Word Bearer warband gets annihilated almost to the last man, because the Thousand Sons believe it was them, not you, that did it.
Back in the Eye, at several points the Black Legion descends on your stronghold. Sometimes, they're weak and feeble, offering you a chance to join them. You refuse. Maybe you negotiate peacefully. Maybe you destroy them. Maybe you sense their leader is a weakling getting above his station, and has nothing to do with Abaddon, as many Black Legion warband leaders surely don't. Other times, they descend in force enough to annihilate your world/fleet/stronghold, and you have to run. Maybe they catch you, though. Maybe they offer you the choice to wear the Black, or be destroyed. Maybe you manage to escape anyway, and maybe your Host is finally destroyed. Maybe you join the Black Legion out of necessity, and despise it, gearing up to betray them later. Maybe you find that the freedom is liberating, and stick with it. Maybe you find that it's literally no different - your warband is still the same, your faith is still the same, and you have the same complicated relationships with Black Legion warbands that you did with Word Bearer warbands. Legion ties mean everything to some warbands, sometimes, and nothing to others, at other times. Maybe you spend endless campaigns devoting yourself to the Black legion, or the Word Bearers, only to find more and more warbands from your Legion are defecting, or betraying you, or never swore quite the same oaths as you did. Why didn't they? Or did they, and it's a misunderstanding? Remember that time you were assumed to be a traitor to your Legion? If they would just stop firing at you and killing your fleet, maybe you could sort it out. Wow, how many men did you just lose in a misunderstanding? Does the truth even matter anymore? If you keep hesitating and bleating about misunderstandings, you're going to be wiped out. How weak do you look in front of your men right now? Kill or be killed.
Say you remain a Word Bearer, though. maybe over time you seek the Black Legion out yourself, to swear loyalty the Legion that seems more unified in your sector of the Eye, or that is enjoying the greatest success and by far the strongest. Maybe you remain a Word Bearer, and the local Black Legion warbands are pathetic. You lord it over them, and demand tribute. Maybe some pay, others resist, and others run to tell Abaddon. maybe Abaddon listens, and descends with the Planet Killer. Maybe he ignores the mewling whines of weaklings, and you never hear from them again. Maybe the Warp eats them, or another warband, or daemons, or some ghost-god of the slain Eldar civilization in whose ruins the Chaos Marines have made their empire.
While you're Crusading next time, your Host joins with two other warbands - a World Eater remnant, and a Death Guard warband. Of the three commanders, you have the most power and influence, so you become de facto leader of this Council of Three. Now you have Khorne Berzerkers and Plague Marines to use in battle. This is the life, right? Except, the three factions of this new warband don't get on. They ally for convenience, or because they recognise the advantage, or even just temporarily for the current Crusade - or even just for this single world, and once it's taken, they break apart. Or maybe you manage to hold them together, and they become your lieutenants. Maybe you keep them in line, despite fights constantly breaking out between the rank and file warriors, and the clash of faiths, and the constant pressure of your lieutenants to undermine you and take control themselves. Maybe you're a lieutenant, and the Plague Lord or Skull Champion leading your mixed warband is weak, or foolish, and you know you can lead better than he can. So you're the one working to take control. You're the commander of the Host of the Sacred Phoenix Unbound, but your warband itself is known as The Triumvirate, and your reputation for success, brutality, cunning and competence begins to cast a long shadow. Other warbands can't match you in size, and begin to pay fealty, or serve you, or simply flee from you when you enter their territory. What was a simple alliance in a Black Crusade is now a legitimate warband. maybe it breaks down in a month. Maybe it lasts 8,000 years of ruthless, elegant destruction of its enemies.
But you remain a Word Bearer. Maybe you remain closely allied with your other commanders in The Triumvirate, but you prefer to spend most of your time working alone, and the bonds of the Triumvirate are only for times of great need. That makes perfect sense. Then it becomes like any other allegiance of warbands, and you go your merry way. maybe you never had anything to do with those pathetic deviants of broken Legions anyway.
Now you need new Marines. You find a sorcerous way to breed them. Or the technical facilities to clone them. Then you have to find the lore to clone them yourselves, or hire out a fallen Apothecary who is absolutely insane, for the right for him to help you. But he wants an artefact lost on on a daemon world, or deep in the Imperium, or he wants his former warband destroyed for exiling him, before he'll help. Maybe he does as you ask, but it doesn't work. Maybe he's too unreliable and screws it all up for you. Now you're even worse off, having lost the facility and all those resources and the time spent getting it all. Maybe he does it, and it actually works.
Or maybe you raid loyalist Chapters and harvest their fallen for gene-seed? Attacking Space Marines is risky, they don't just sit around and wait to be attacked. They strike planetary targets hard, then leave. They rarely defend, and usually attack. How do you find them? En route to a world? Lure them in? What if they bring overwhelming force and you risk destruction?
Then, of course, what about purity of bloodline? Does that matter to you? Does it somehow affect you on a deep level if your own future brothers are made from Imperial Fist or Ultramarine gene-seed? What will you do? Maybe you attack other Word Bearer Hosts and harvest their gene-seed. Maybe you cut deals with Mechanicum factions - which themselves are devolved into countless city-states and independent worlds in the Eye, allying and oathbreaking in much the same way the Chaos Marines are, and the same way every faction in 40K does to a lesser extent. Maybe you arrange to protect their forge world in exchange for them establishing gene-seed and Marine production facilities. What if they're invaded and you can't handle the scale of the war, though? Maybe you run, and earn the eternal enmity of the local Mechanicum. Maybe you get away clean. Maybe you fight it out, and suffer horrendous losses. Maybe you win easily, and the allies of the destroyed warband come for your head. Maybe the Mechanicum world you've agreed to serve screw it up, and you've just wasted a lot of time, ammunition and Marine lives in a campaign that profited you nothing.
You could fly to other Word Bearer territories, and ask to use their facilities. Maybe they welcome you with open arms. Maybe they've heard you're a traitor, or that you refused to come to the aid of another Host, or that you sinned in some way that you've never even considered or heard of. Maybe a greater Legion commander steps in and weighs in favour of your claim to gene-seed facilities. Maybe you're cast out, and have to sail elsewhere to deal with others. Maybe you approach one of the major power players, like Erebus or Kor Phaeron. Are they on campaign, in the Imperium, for countless decades? Are they on Ghalmek or Sicarus? Reaching either of those worlds - reaching any world in the Eye of Terror or the Maelstrom - where space and time obey no physical laws - may very well be something like the Odyssey, a journey that could take days or decades, under the eyes of malignant daemons through a realm that itself exists to possess, mutate, consume, and digest material life.
Maybe the human thrall-army that serves you mutates beyond usability. Maybe they're lured away by another cult's faith, to serve another warband. Maybe they're just slaughtered in a bad battle, and you're massively underpowered in terms of Traitor Guard and human slaves now. Raid prison worlds? Hmm, good body count, but they're untrained. Try to convert Guard regiments? Tough call, and it involves careful sedition work, and your only trusted co-leaders spending years away from the warband to convert the human armies. Maybe you could send your less trusted underlings to do it, but then who would the new humans serve? Would they be an army secretly waiting to betray you?
Maybe you agree to serve a Black Legion warband for sixty-six years, to have your Apothecaries and Fleshsmiths trained in the mastery of Berzerker surgery. Now you have Berzerkers. Awesome. But what if another Word Bearer warband considers that a dangerous collusion with an enemy Legion, despite the fact you've been perfectly loyal and you know a dozen other warbands have done the same? Now you have another fight on your hands, with fanatics as dangerous as you are. Maybe the Black Legion warband you've allied with betrays you, and reveals nothing at all. Maybe they're destroyed from internal conflict and the new leader refuses the old deal his predecessor made. Maybe over time, your Host and the Black Legion Company become bound in blood-loyalty, and form a joint warband of two factions. Maybe you get tired of them after three days and replenish your recent losses by stealing their ships in an ambush.
What if Vilus doesn't die? What if he starts grooming other co-leaders as the warband grows in size? Maybe he means to replace you. Maybe you most loyal warriors keep telling you they know a betrayal is coming, that the whole warband may have to choose sides. Maybe you act first, and start the civil war. Maybe Vilus does. The warband breaks apart, and the survivors go their own ways. Maybe both warbands call themselves the Chapter of the Sacred Phoenix Unbound. Maybe you surrender the name, or Vilus does, because he no longer cares about the old traditions.
What if Vilus encourages contests of competency between his underlings? You lose favor by failing wars, or gain favor by successful missions. it's all a game to him. He's no leader at all. You could command the warband and focus it to better, more meaningful ends. Right? Right? It's not like almost every other Aspiring Champion in the warband thinks the same thing, after all. Right?
While you're away raiding the Imperium, what happens to your stronghold? Is it hidden well enough in the Eye of Terror? Maybe it is. What if time and space distorts and reveals it, though? Do you leave a garrison of your best troops to defend it? Maybe you do, then you take the battlefield without your best warriors. Maybe you return and another warband has still destroyed your base, or occupied it for themselves. So begins a costly war to retake your fortress, or carry on as a fleet-based warband, forever suffering a lack of resources compared to those with fully operational and well-supplied homeworlds. Maybe you go to Sicarus as just one of the Hosts using it as a base. You still share the territory with countless warbands that have an eternity of complications, grudges, oaths, betrayals and future treacheries on their consciences. You're certain half of them even here, in your Legion's deepest claimed territory, are only united by strong leaders that demand they stay united, and enforce it by death. So many warbands seem honestly bound by brotherhood, but look over there - the fleet of the Host of the Shrieking Wound. You know they failed to come to your aid a few centuries ago, and betrayed the Legion at the Battle of Ziar by taking too long to commit to the fight. Maybe they say the same things about you. Tension is always rife, even between blood-bound allies, for glory in the eyes of the Legion's power players, and in the eyes of the Gods.
Maybe you stay. Maybe you go. Maybe, maybe, maybe.
That's what it's like to be a Chaos Marine.