Now we have the fluff and background of this new unit. I have taken the background, the information from earlier, and the latest additional bits and put them all in one location. So have fun.... Your going to enjoy this one.
via Faeit 212 (a Must Remain Anonymous Source)
Hellbrute (background/fluff) paraphrased
They were mighty and honorable warriors interred in the powerful and venerable dreadnought chasis during their former slavery to the Imperium. A side effect of insanity was a known feature of Dreadnoughts. To counter this, standard operating procedures on Dreadnoughts is to keep them in stasis between service, so that they see life as one constant battle, only awake in times of great need and focus, but Hell brutes get no such reprieve.
Without the cold embrace of nothingness that makes the years and decades between war pass in the blink of an eye, they are driven mad. The rage of a mind essentially trapped within his walking sarcophagus, simultaneously his freedom and prison tears at the very fabric of his sanity and it is expressed through monstrous transformation and mutation.
Some believe that when enough service has been rendered and enough blood has been shed and victories won in the name of the Dark Gods, these once mighty warriors will be elevated to Greater Daemons and once again walk the earth of dead worlds with their own two feet.
I just want to clarify that while it is listed as having a Power Fist and not a Dreadnought Close Combat Weapon, because it is a Walker it does not suffer from the Unwieldy special rule, that's why it's simplified in the codex entry.
Str 6, ws 4, bs4, i4
Multi-Melta / Power Fist (with inbuilt combi-bolter)
can swap the Multi-Melta for a butcher-cannon, or twin reaper auto cannons (which is pretty cool, since that's an entire riflemen in one arm, but with less range), twin-linked lascannon or a twin-linked heavy flamer.
can swap combi-bolter for a meltagun or heavy flamer
Mark of Khorne: Rage USR
Mark of Nurgle: 5+ restore a hull point OR repair immobilized / weapon destroyed at the start of a turn. Does not let it get back up after having been killed unlike Unholy Vigor.
Mark of Slaanesh: Sonic scream, gives it offensive/defensive grenades
Mark of Tzeench: flame weapons (if it has any) are +1 strength and -1 AP (heavy flamers are AP3... whaaaat)
Some stuff I left out (it was late, I was tired)
You can change the shooty arm for a Power Fist. The power fist doesn't have an inbuilt weapon though so you can't get another flamer or meltagun on it. This upgrade is free.
The autocannon is low points cost, the butcher cannon is moderate, the lascannon is expensive and the twin-flamer is free.
It's also still crazed, but crazed works differently now.
If you get fire frenzy, you double shot at the closest enemy target, not closest target. Only if there are no enemies in range do you shoot a friendly target (twice). If nothing is STILL in range, you treat it as blood rage instead. (so upgrade your stuff to flamers and give it another power fist if you always want it ANGRY)
Blood Rage makes the Hellbrute have to move towards the closest target. Rolls 2d6 to run (pick the highest) and can charge, and must charge, the closest target within 12". (rolled as normal). If immobilized, you treat as a Fire Frenzy.
Dark Apostles make Blood Rage results more likely to happen if they are nearby (but you risk the albeit now unlikely fire frenzy to the face).
They're not infected with the same virus as Obliterators.
It's not a daemon, and does not have the daemon USR
The fluff entry also mentions that some Hell Brutes are not as mutated as others, and more closely resemble their dreadnought kin, but to make no mistake they are just as insane.
(So you can still use your old models if you have them. The hellbrute is just the evolution of the whole "style" for the new CSM, which is a lot less "space marine with spikes" and a lot more something unique).