Rubric Marines
"Immortality is a terrible curse."
-Simone de Beauvoir
And I will show you something different from either
Your shadow at morning striding behind you
Or your shadow at evening rising to meet you;
I will show you fear in a handful of dust.
-T.S. Eliot - The Waste Land
Rubric Marines - the cursed automatons of the Thousand Sons, and one of the two toughest infantry units that are available to the Traitor Legions, right alongside their Nurglite rivals, the Plague Marines. They serve a similar tactical role in that they're absolutely tough as balls, but whereas the Plague Marines, owing to their diseased bulk, can resist large volumes of anti-infantry fire, the Rubric Marines, protected by dark sorcery, will shrug off even the most powerful hits from anti-tank weapons.
They've been known to get pre-5th Edition Necron players to rage-quit.
Lore
The Rubric Marines were once basic Marines in the Thousand Sons. Due to someone's fuck-up with their gene-seed, the legion as a whole suffered horrifying, increasingly-debilitating mutations. For a time Magnus managed to stop this shit by striking a bargain with Tzeentch (for which he lost his eye BTW), but this worked only until the start of the Horus Heresy. This generally sucked, since it meant that increasingly, members of the legion were turning into that which must never be named. Tzeentch, in the Chaos God's proud tradition of not giving a fuck, left the Sons to work out their own solution to the problem, likely so he'd have some entertainment over the weekend.
At some point, trying to help fix this little mutation epidemic, a Sorcerer called Ahzek Ahriman used the knowledge he took from the Book of Magnus (which he was a bearer of), to find a way of curing his legion. He crafted a way to undo the mutations plaguing the legion, and cast an incredibly powerful spell called the Rubric of Ahriman. The spell was successful in stopping the mutation, but only really amongst the command staff - every non-Sorcerer in the legion was reduced to dust and trapped as bound spirits within their sealed power armor for eternity (or at least until the armor was destroyed). Suffice to say, Magnus was really pissed off and wanted to blow Ahriman up with mind-bullets, but Tzeentch asked him not to because that was some pretty epic douchebaggery. So Magnus kicked him out of the Thousand Sons, instead.
What a dick.
A huge part of the Ahriman's initial post heresy activities was trying to reverse the effects of the Rubric by, somewhat counter-intuitively, performing the Rubric again but doing it right this time. To this end, he created a new Rubric, and broke through to the Planet of the Sorcerers in order to perform it. Due to a huge amount of Just As Planned dickery from a number of parties, it didn't work. With the exception of one marine who was returned (sans any personal memory) to a full normal body. Rather than being declarative proof that the Rubric can be reversed, it seemed to be a concession on the part of Tzeentch to give Ahriman a little hope and string him along for more giggles further down the line.
Since the Rubric of Ahriman, the Thousand Sons have lost many of the Rubiks Cube Marines, but they still manage to keep their numbers at near thousand. Two main theories that they either perform the scaled-down version of Rubric on other marines, or reanimate fallen ones by pulling their souls out of Warp and putting them into repaired/new sets of armour (The latter theory has been confirmed by Phil Kelly, but he also mentioned that the Black Legion uses the former version to make their own rubric marines). Or both. Whichever is creepier is up to you to decide. It's ironic, as the fluff states, they were turned from a legion of scholars and book lovers, into mindless, walking power armors that can't even think!. They can think, its just that they've lost all hope and purpose in life. Reduced from scholars to beings who can't even feel, they let the days slip into millennia. Only battle barely reminds them of what it was to be alive, and they fight accordingly. So grimdark.
They're also central to this tomfoolery, so props for that.
On The Tabletop
Rubric Marines aren't quite as good at ignoring the sustained fire of 40 Shoota Boyz as their counterparts, the Plague Marines are, but unlike them they can ignore a direct Battle Cannon shot, since Rubrics possess a phenomenally-dickish 4++ invulnerable save. Even better, Rubric Marines pack Inferno Bolts, which means their Bolters are AP3. Suffice to say, they can carve through a squad of MEQ effortlessly, and, prior to their massive Tomb Kings in Space update, fielding one squad of intelligently-managed Rubric Marines essentially guaranteed victory against a typical Necron player (or current-gen Tyranid player; thanks a fucking bunch, Cruddace). They're also led by a Sorcerer, who gives them access to one psy power (typically something like Bolt of Change, so the squad can pop transports). The Sorcerer's Bolt Pistol also has Inferno Bolts.
Rubrics are cursed with Slow and Purposeful, and unlike the other aligned Chaos Marines, do not have close combat weapons or bolt pistols. At a glance, this makes them a very easy-to-target melee weak-spot, but in practice, this isn't quite true; they still have their beefy-as-shit 3+/4++ saves, and the Sorcerer's Force Weapon means that no MC worth their shit will get anywhere near them. What they are vulnerable to is being Tarpitted, due to their relatively lackluster number of attacks and immense focus on shooting to take advantage of their juicy AP 3 Bolter shots. Note that whilst they lack grenades and thus hit last if they (counter)charge into cover, they still have a marine statline for close-combat and a 4+ invuln save besides, so seeing these guys win a fight with a squad of Banshees isn't as absurd as you might think.
They also don't have grenades, so charging through a cover or blowing tanks with krak-grenades might be a problem, and even more so if they got caught in melee with AV11+ walker, since they're fearless and cannot choose to run away.
Rubrics benefit heavily from having a Rhino Transport. Not because they need the survivability (they're tougher than the vehicle they're in, to be frank), but because the mobility they get is such an enormous boon. Take full advantage of their ability to shoot out of their transport, and use it in glorified Tzeentchian drive-by shooting, before disembarking in rapid fire range and warpfire-blasting a bitch.
Mainstream opinion
Rubric Marines are garbage, absolute, complete garbage. If you are taking them you are guaranteeing your defeat. Like many many Chaos units, they have good looking stats but are too expensive to be worth taking. Even with a 4++ they're only MEQs, who also are shit in close combat, and can't run. Their main job, with AP3 Bolters, is to hunt down other MEQs and with only S4 and no means of bypassing reanimation protocols or feel no pain they aren't really amazing at it and remain frightfully ineffectual against Necrons and Nurglites and are of course; poor against monstrous creatures, vehicles, and TEQs. The thing is, basically any other expensive unit can kill MEQs far better than Rubric Marines can. You could get a lot more bang for your buck by putting points toward a Daemon Prince instead, who can straight up liquify MEQs, and just about anything else too. The sorcerer that comes with the squad is also pretty lackluster, since he's forced to roll his powers from the Tzeentch table, easily the worst out of the Chaos Space Marine specific disciplines. In the end you're paying for a MEQ killer that's outshone by just about everything. They're also finecast.
To make matters worse, Rubric Marines lack any means to deal with cover, so all their shootiness goes out the window the moment the enemy puts his MEQs behind cover, which is all the time. And they are much worse per point at dealing with GEQs who are going to be in cover anyway, Soul Blaze might seem like it can help but it won't because Soul Blaze is the most ineffectual USR of all time and more often than not mostly just serves to make you more useless against certain units. Due to having no extra rate of fire or strength per shot compared to regular Chaos Space Marines and lacking any special weapons at all, Rubrics are literally only better than a Chosen squad with plasma guns at killing Marines standing in the open for their points and basically nothing else in the game, and with the general finickyness of psychic powers in 7th edition and the overall terribleness of the Chaos Space Marine Tzeentch table the Sorcerer won't do much to improve their lethality and can be shut down entirely if your enemy's got more psychic game than you do. Which they are quite likely to have unless you're running a mono-Tzeentch list (in which case you probably don't care about winning because mono-Tzeentch CSMs are terrible).
Rubrics melt when exposed to weight of fire just as easily as regular marines do and thanks to their high cost you won't have as many bodies to soak up the punishment your marines will suffer. So expect them to be gone when those Ork Shootas, Guardsmen Lasguns, Fire warrior pulse rifles, Necron Tesla blasts, or just about any form of Tyranid shooting gets their hands on you. Sure they can tank MEQ killing shots better than most marines and their anti-MEQ killing save can't be stripped away like cover saves, but nobody is going to care because most serious lists in 7th edition can put out more than enough dice to mulch your rubric marines' overcosted ass. And it's worth reiterating that they're all going to die the second they're put into melee because they are firewarrior level terrible for their points cost in melee> Fucking grots will tie them up all game long, if some flayed ones get into assault with them you can kiss that whole squad goodbye.
And no, they're not all that great against Necrons because Necrons will tend to have cover saves and Rubric Marines can't do anything about the reanimation protocols that Necrons get. Meaning that they're not even very good against one of the most heavily used top tier armies in the game. And while they might be rather resilient against Eldar shooting with their invulnerable saves, they certainly aren't going to survive tangoing with Striking Scorpions or the huge weight of poor AP but high strength hits the Eldar can lay down on them, and you can probably just forget about getting to use your sorcerers against the Eldar and their psychic spam. Oh and against AdMech lists? You might survive the grav guns and the pieplates, you probably won't survive anything else the Adeptus mechanicus can shoot you with. So yeah, like just about everything Tzeentch related on the tabletop regarding the Chaos Space marines for edition after edition they're garbage, leave them at home and never even think about bringing them to any competitive games.
So Then "Wrath Of Magnus" Came Along...
Rubric Marines got a rules update along with the general Tzeentchian overhaul in the new Chaos supplement that was part of the War Zone Fenris pack. While not too much has changed about the roles and rules for the Sons, they are a somewhat better unit than before. Rubric Marines now have weapons options, which is a refreshing change. They have access to AP4 flamers with Soul Blaze (and contrary to popular opinion, Soul Blaze isn't bad, it's just not great), so they have something to counter cover but at the cost of range (and still no Overwatch). They also have access to a S5 AP3 Assault Cannon, which is actually pretty cool. Also the new Discipline of Tzeentch makes the Sorcerer less likely to be unuseful, although the new discipline can be prone to random rolls and FUN. Where they really shine is their their new formations, which gives bonuses to manifesting and casting powers or rerolling saves. On top of all that, they have slick new models.
This all rounds out to a unit that's okay-ish, on paper. Rubric Marines are still an expensive unit that fills out a considerably specific role in the army, and while they've been given a degree of flexibility, they still more or less have the same strengths and weaknesses. The key to using them is still managing them intelligently (even knowing when to leave them at home), keeping them to their strong points, and synergizing them with the rest of the army.
And We Also Have Terminators Now
Rubric Terminators. Rubric. Terminators. Have you read the Horus Heresy novels, noticed that the Thousand Sons have these sweet ass, psychic, foreshadowingly stoic Scarab Occult Terminators, and you ever wonder what happened to them? Apparently, the Thousand Sons still had them all this time, but Magnus had stuck them into the bottom of the pantry to save them for a rainy day. War Zone Fenris was that rainy day.
While the anti-Rubric Marines group are probably groaning at the moment, they're actually pretty good. You'd probably expect the 2+/4++ saves, Fearless, and AP3 combi-bolters, what you weren't expecting was the lack of Slow and Purposeful, one of Rubric Marines' chief weaknesses just gone. You probably weren't expecting the Aspiring Sorcerer to keep the good stuff and be upgraded to ML2 (which means the option to roll a power on a second psychic discipline), and 2 wounds. They also have weapons options for a shorter-ranged Cyclone missile launcher that shoots kraks, and AP3 heavy flamer, or that odd Assault Cannon analogue mentioned above.
However, they still share in the Rubric Marines other traditional weaknesses. No combi-weapons and no AP2 power weapons means that they're still anti-MEQ specialists with limited options for countering cover, TEQs, and walkers. And with a minimum of 5 guys, the're still not a cheap option.