"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.
The Rubric Marines were once basic Marines in the Thousand Sons. Due to
someone's fuck-up Tzeentch’s tampering 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 psyker in the legion who was not powerful enough (virtually all of the Thousand Sons were psykers according to recent fluff) 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. Interestingly, in recent fluff it is implicated that Lucius may have altered the Book of Magnus after making Haathor Maat his bitch. He gained his trust by showing him (as he was starting to succumb to the flesh change) how to reverse the mutations simply by pumping them all into worthless mortals so he would not become that which is not to be named. So it may well be that Lucius' dabblings with the Book of Magnus and changing it resulted in the Rubric to fail. For whom or why he did it only he knows. Or he was a temporary agent of Tzeentch at the time (JUST AS PLANNED).
A huge part of 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 nearly a thousand. Two main theories that they either perform a scaled-down version of the Rubric on other marines, or reanimate fallen ones by pulling their souls out of the 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. Additionally, the dust containing the essence of the Rubric Marine can be recovered and be put into a new set of armor, effectively resurrecting them. 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.
In the Gathering Storm II, it's revealed Yvraine can REVERSE the rubric. She does in a bid for Ahriman to save her, so she won't be driven mad by Tzeentch. This has an emotional effect on him, but the sneaky Eldar blows a hole in the webway, and all the un-rubric'ed marines get sucked into the warp. This trolling of the poor Ahriman (who goes into nerd rage so hard even Khorne approves) is so great that Cegorach decides to lend his support to Ynnead. What a bunch of dicks.
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 Inferno Bolters are AP2. 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 Inferno 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 while 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.
Sweet Jesus, the edition update from 5th to 6th and 7th was not kind to these guys.
While Rubrics were always a finicky option, one that had to be balanced carefully to be used to maximum effect, 6th and 7th wound up indirectly killing their potential on the battlefield by giving them way more competitive options and relegating them to a much more limited tactical control. Their AP3 Bolters and 3+/4++ saves, while once rather impressive, no longer have anywhere near the threat level they used to have, and with other armies getting major edition updates, the Rubrics were facing an uphill fight - especially since changes to the rules made Feel No Pain considerably stronger (ergo pushing Plague Marines even further ahead of their old Tzeentchian rivals. The changes to Sorcerer rules and Force weapons were even worse - taking a unit that was legitimately a threat to one that struggles hard for a place in a meta with dozens of options. Even within chaos a player could grab a baseline squad with two plasma guns and have better numbers (though none of the backup save) as part of the bargain!
But the cruelest cut - came with 6th edition's addition of the ability to go to ground. This meant absolutely any unit could gain a cover save, basically anywhere, and cover is one of the few things that, Psy Powers aside, the unit just has no ability to deal with, and since the Sorcerer was usually angling for Bolt of Change to kill transports, that's a pretty glaring vulnerability for a unit with a very specific tactical role.
Worse, errata made it so Sorcerers had to take powers randomly with rolls as opposed to choosing them, which seriously cut down on the strategic elements of using this unit, since you no longer had the utility of that lone bit of choice giving you the option to use the squad more tactically based on said choice. Their weapons were also given Soul Blaze, a sub-par ability that really struggled in earlier editions, further weakening their role on the battlefield.
So no more instant kills from the force weapon, mediocre-at-best weapon ability in sorcerer, can't choose psy powers anymore and has to randomize, squad still same drawbacks, in a world that gives everyone cover saves and pushes vehicles harder than ever before. They were also Finecast, because fuck you.
This combination of factors took what was previously a fairly effective unit against MEQs, holding terrain, and terrorizing medium infantry - and completely demolished its tactical role, turning it into an inefficient unit that struggled for any relevance against a backdrop of other chaos units doing the same thing better. With factions that were previously terrified of this unit no longer really having to even worry about it, Rubrics fell into a steep decline, and within a year and a half of the edition update, even players who lived and breathed Thousand Sons stopped using them, and the unit essentially tapered off, unused by even its most ardent fans.
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.
Once unusable, these guys have become pretty good (but overcosted). They have their usual loadout of Inferno Boltguns, which they can replace with Warpflamers for maximum hurt in overwatch, and the Aspiring Sorcerer can take any force weapon and a Warpflame Pistol to help them if they get in melee. For every 10 models, one Rubric can take a Soulreaper Cannon or a Heavy Warpflamer. The Sorcerer now knows one power from the Change Discipline and Smite, which does 1 or d3 mortal wounds rather than the normal d3/d6. However, the best part is their "All is Dust" rule, which gives them +1 to saves (including invulnerable saves) if the attack does 1 damage, giving them a 2+ save against small arms fire. Thousand Sons and Grey Knights are the only armies who can still spam smite, but you should be very careful doing it with regular Rubric squads because if you peril their sorcerer may explode, killing himself and taking along with some unlucky mooks from any nearby units, friend or foe so you might start a chain reaction (as well as preventing the squad from casting any more powers if they survive).