Rubric Terminators. Rubric. Terminators. Have you ever read the Horus Heresy novels and noticed that the Thousand Sons had these sweet ass, psychic, foreshadowingly stoic Scarab Occult Terminators that were literally described as "automatons" and "like robots" by Jaghatai Khan and Ferrus Manus (although the latter was most likely a compliment)? Did 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.
Why did they get Rubric'd?
You may have noticed the Scarab Occult was originally composed of the legion's best and most disciplined psykers. So, if Ahriman's Rubric transformed astartes who had little to no psychic abilities into dusty power armors, why did most of the Scarab Occult also get the Rubric treatment? Because the Rubric affected everyone but the most powerful psykers. Meaning that only a few of these guys managed to power through it, while the rest turned to dust because they just weren't powerful enough. That and Tzeentch likes ambition. These guys were automatons in all but name, who loyally served Magnus and...that's it really. Tzeentch no likey.
On The Tabletop
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 and resulting in the odd scenario of Tactical Dreadnought armoured Marines being faster than regular Power Armoured Marines. 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 Hellfyre Missile Rack that shoots kraks, and AP3 Heavy Warpflamer, or that odd Soulreaper Cannon 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. The lack of options to negate cover saves is a particular issue as the targets you'd want to use them against the most; MEQs and Necrons, are almost certainly going to have cover saves (and inferno bolts do nothing about RP rolls or the FNP of Slaaneshi, Nurglite, Iron Warriors, and Iron Hands marines or the added toughness of Nurglites) and they are very pricey for the rather mediocre firepower per model they bring to the table. The AP3 heavy warpflamer is nice for roasting cover camping marines but it will require deep striking them to point blank range to get the most use out of and you still have to pay for the other terminators. However with two wounds they're pretty good at soaking up large volumes of fire thanks to their Rubric Terminator Armour; requiring an average of 38-39 lasgun shots to kill and with their invulnerable saves and their two wounds they can tank plasma fire decently well; requiring roughly eight shots from guardsmen to kill and if you can get Endurance on them from some source they can be amazingly resilient to the point of nigh invulnerability. In conclusion their firepower is overall quite mediocre but their durability is top class among TEQs; though against the likes of meltaguns and lascannons they aren't anymore resilient than any other T4 model with a 4++ save.
For some reason even though the Forge World 30k line shows them in Cataphractii, in 40k they're in Tartaros.