The intoxicating lure of Chaos leads untold millions down the path of corruption, and the noblest of intentions offers little protection against its dark influence. A soldier’s bloodlust, a politician's ambition, a lover’s desire, and even a mother’s whispered prayer over her feverish child – they are all the playthings of the Dark Gods. Why resist? Wealth, power, and happiness await those who serve the Ruinous Powers, and the only price is your humanity.
Alternative Character Interpretation: Within the context of the setting, the Imperium can be viewed as a necessary force to safeguard and shepard humanity against the darkness which threatens to consume it, or it can be seen as a brutally oppressive regime which holds humanity back from realizing its true potential. A writer's sidebarearly in the rulebook makes clear that Black Crusade is written with the later perspective in mind.
Anti-Hero / Anti-Villain: This game allows you to play this as well as the shallow-one-dimensional cackling Card-Carrying Villain. As the opening quote shows, sometimes the first tragic step in becoming a monster can be as noble and pure as a parent's love for her child.
Black and Grey Morality: While Chaos is generally Always Chaotic Evil, an often overlooked aspect FFG is trying to re-emphasize is that Chaos also embodies positive concepts. Khorne, for example, is not only the god of bloody madmen, but also martial champions constantly defying the odds and looking for an honorable challenge.
Body Horror: As a heretic becomes more and more corrupt, he body will slowly start to mutate, and some mutations are rather...impressive.
Byronic Hero: A viable character option. When they turned to Chaos, many Heretics felt the Imperium is far too oppressive and limiting for the good of humanity, especially when they feel that embracing the powers of Chaos can help humanity preserve itself in a galaxy trying to kill them. Sometimes it's a more personal level than that, but for every Lord Byron that serves Chaos, there's probably more who are serving the Ruinous Powers for more selfish goals.
Class and Level System: Notably averted. The followers of Chaos are a diverse and fluid lot, and so Black Crusade characters are not restricted by the same Career Path advancement system as their Loyalist counterparts.
The Corruption: Given Black Crusade's focus, this is explored in more detail here than in previous Warhammer 40,000 RPGs; in particular, the Corruption track is intended to be used as a sort of "progress meter" for the campaign as a whole, tracking how close the PCs are getting to becoming DaemonPrinces or ChaosSpawn.
More specifically, there's actually two tracks in the game: actual Corruption, and Infamy, which is the measure of both how much you've impressed the Ruinous Powers and your ability to control your mutations. Hit the threshold where you lose your humanity with a high enough Infamy score, and it's presumed your patron blunts the impact of the runaway mutations long enough for you to control them, and you become a Daemon Prince. Otherwise...
Fat Bastard: Assuming a chacter isn't this already, a possible mutation is that characters will become quite large. Followers of Nurgle have a chance of this being taken Up to Eleven.
Fragile Speedster: Human Heretics lack the inhuman resilience and Powered Armor of Chaos Space Marines, but all possess an ability allowing them to act sooner in combat.
Gameplay and Story Segregation: In theory, Black Crusade lets you play a veteran legionnaire of the World Eaters, Death Guard, Thousand Sons or Emperor's Children, who has presumably been a loyal servant of your chosen Chaos god for the better part of ten thousand years. In practice, it's not actually possible to build a starting Chaos Space Marine who isn't Unaligned, and in fact the character must rack up a significant amount of in-game experience before he can formally align himself with one of the four Great Powers.
FFG seems to address this issue in the upcoming supplements involving the various Gods of Chaos. The first, Tome of Fate, has been confirmed to contain a Thousand Sons Sorcerer archetype that begins play aligned with Tzeentch.
Great Escape: The Free RPG Day introductory adventure, "Broken Chains," revolves around the PCs attempting to escape (or conquer!) the compromised Imperial prison barge Chains of Judgement.
Mad Scientist: Having traded the hidebound scriptures of the Adeptus Mechanicus for the freedom and insanity of Chaos, hereteks tend to be both better scientists and crazier madmen than their erstwhile tech-priest brethren.
Magitek: This is the bread and butter of Q'Sal, world of the sorcerer-technocrats. Their Crystal Spires and Togas society is kept aloft by daemonic machines and sorcery. It also states something about the setting that the people constantly exposed to The Corruption of said tech reign over a planet with living conditions better than the vast majority of Imperium worlds, combined.
Must Have Caffeine: Hilariously, in a game full of Chaos Marines, mutants and daemons and about a journey down a dark path to damnation and ascension, getting addicted to "recaf" is still a valid condition.
The Heretek archetype seems to have this on full display-the description makes them seem like actual scientists who got fed up with the Adeptus Mechanicus' restrictive attitude, and you can almost hear the statement of "most of [their research] tends towards war" ending with "because they work for Chaos, and that's what it pays them to do".
This kind of Heretic is embodied by Captain Korginthe World-Reaver, one of the example rivals for the PCs. Despite what his many titleswould imply, he actually worships Khorne as the embodiment of martial honor. Thus, he honors pleas for mercy (as he thinks only the skulls of people who fight back are worth sacrifice), and pointedly avoids some of Chaos' Kick the Dog tactics.
One of the example origins for the Apostate is basically this. A loyal priest of the Emperor who ministered to the Imperial Guard and got sick of watching millions go to their deaths and thus turned to Chaos in order to sabotage the Imperial war effort.
No Points for Neutrality: Somewhat. Staying Unaligned denies a character access to powerful Gifts, items and psychic powers and means they don't get discounts on certain skills, talents and attributes. However it also means they don't have to pay extra for purchases opposed to one deity or another and, for high level psykers, grants them access to powerful Exalted powers.
Non-Action Guy: The ready availability of combat-capable Minions makes this a more viable build option in Black Crusade than in the other 40k roleplaying games. A player can easily build his Heretic to be The Smart Guy or The Chick (or the Evil Genius or Dark Chick, as the case may be), whose only viable option in combat is to hide under the nearest rock until the shooting stops, and so long as the Heretic has a Minion or two around to serve as a bodyguard, that player will still have something to do in combat and need not worry about being left out.
The Pig Pen: Followers of Nurgle can become this, becoming filthy and destructive, and also contemptuous of beauty and the fine things in life. Another manifestation is that things around him seem to break down while he (and he specifically) has them.
The Renfield: Black Crusade characters can take Minions, followers of fanatical loyalty (but sometimes questionable competence) that can fill holes in their master's repertoire, or simply provide a source of warm bodies to throw at opponents. Taking multiple iterations of the Minions Talent can eventually lead to a character acquiring his own Quirky Miniboss Squad.
Sanity Meter: Averted, unlike in the loyalist-centric 40k RPGs. This is because any Heretic worth his salt is assumed to already possess 100 Insanity Points by the time play begins.
Sense Freak: This is a pretty heavy theme with Slaanesh. There's also a drug called rose which heightens sensory input to incredible lengths, that the user can see in the dark; but the user runs the risk of sensory overload.
Small Name, Big Ego: In-Universe, Tzeentch followers can become convinced they are incredibly powerful psykers. This makes them incredibly irritating to deal with, especially for real psykers.
Super OCD: Characters can develop this, followers of Tzeentch would manifest it as a paralyzing fear of contamination, while followers of Slaanesh can't focus unless something is "just right".
Tragic Hero: Many join Chaos with the noblest of goals and the best of intentions: to protect those they love, to make the world a better place, to stand against the tyranny and cruelty of the Imperium. Many fall. Many more die trying. Hardly any of them live to see their original dream fulfilled, in a form they would still recognize, and remain uncorrupted enough to care.
The version of Imperial history and organization presented here is stripped of the usual implication that it's all justified and things were fine until the Horus Heresy. It really plays up the idea that PCs in the other lines are tainted by evil just for defending it. A repeated theme is that many heretics embrace Chaos just because it opposes the Imperium and have no real loyalty to it.
Weak, but Skilled: Black Crusade balances Chaos Space Marines against ordinary human heretics by granting the latter a greater variety of starting Skills and Talents and a greater amount of starting Experience Points. Of course, when the Talents include such things as "is a psyker" and "can shoot lightning from my nipples," the terms "weak," "skilled," "ordinary," and even "human" begin to lose some of their meaning....
have three levels of objectives: Primary, which must be completed for the Compact to be successful; Secondary, which must be completed in order to complete the Primary Objective; and Tertiary/Personal, which represent each Heretic's personal stake in the Compact. The book recommends that for each Compact, the GM should make some Heretics' Tertiary Objectives incompatible, mutually exclusive, or even counterproductive to the success of the Compact as a whole, so that hilarity (and interesting roleplaying) will ensue. (Thankfully, the book also offers tips on how to stop the game from collapsing under the weight of all the intraparty conflict.)
Wretched Hive: The Screaming Vortex is a wild and lawless region of space. On the outer reaches, the laws being ignored are those of the Imperium and of common human decency; closer to the center, the laws being ignored are those of physics.
On the outer reaches...except for Q'Sal. There, they just ignore physics, as the planet itself is in much, much better condition then the vast majority of the Imperium. And they're Heretics, too. See why Chaos can be so appealing?