Rubric Marines
Alright, the Rubric Marines. What's up with them? Well, they once were normal pansy marines of the Thousand Sons who suffered from horrifying mutations caused by the fact they were from a world with many psykers of every kind, marking warpstuff rub off on every of society's members. Or because they were pawns of the guy who gets all the lulz.
So, they were pretty much screwed, and then a Sorcerer called Ahzek Ahriman cast a mighty spell (Epic moment) called the Rubric of Ahriman! This spell however was pretty fucked up or it just went wrong somehow. Or not.
What exactly happened, you ask? The entire Legion, expect for their badass Primarch and the Psykers were turned into walking husks. Their bodies turned into ash and their souls were bound on their armor. They became silent, walking killing machines who aren't Necrons, now look at that. Oh, and the psykers? Those bastards got their power levels maxed to 9000 and pretty much didn't suffer a thing.
I don't get it
Then read the upper text again. Well, here's some additional crap:
The fact is, Rubric Marines are still capable of following orders! That's right, they can follow orders! But only if it comes from a Sorcerer. And it's the same thing with the tabletop: You got to have a live Sorcerer with them or they're screwed. And they're also screwed in close combat, and do you know why? THEY HAVE ->ONLY<- their Bolters! That's right! They have no chance in close combat, but at range? They're killers.
While being hilariously expensive they are pretty useful, and with the Mark they're even MORE expensive. But they do have 2 Wounds and they're also fearless, so yeah.
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! Just as planned.