Ferrus Manus
Ferrus Manus was the Primarch of the Iron Hands Chapter of Space Marines, HIS HANDS ARE MADE OF IRON! His name is also Latin for "Iron Hand". Real subtle, GW. He got said hands after wrestling with a silver dragon on the planet he crashed on and dripped the lizard into magma and then the metallic skin of the dragon fused into the skin on his hands. Wait, living metal? That sounds oddly familiar for some reason... Anyway, his arms became so fucking hardcore that he did not even *need* a weapon to tear through dudes: his iron hands were more than enough.
So in summary, Iron Hands, who had iron hands, was the leader of the Iron Hands, who give themselves iron hands in honor of the iron hands of Iron Hands.
He also got really pissed off if you called him THE HANDS OF FATE after that incident.
Ferrus, using his shiny metal hands, could make epic weapons that absolutely kick ass but he only used like one or two of them. It's not clear as to whether he or Vulkan was the best smith, but Ferrus certainly had the better way of making weapons, by punching them into shape.
History
Back in the days of the Great Crusade (before the good ol' Horus Heresy), Ferrus met Fulgrim of the Emperor's Children beneath Mount Narodnya, and challenged him to forge the best weapon that he could. After three months, Fulgrim forged an ornate warhammer, while Ferrus made a big blingy sword. After praising each other for their craftsmanship, the two exchanged weapons and became best friends.
Until Fulgrim found a new boyfriend/girlfriend/hermaphroditic freak. Then decided to screw over Ferrus, Fulgrim totally outmatched Ferrus with his HAWTNEZZ and Ferrus jacked the sword he made for his ex-boyfriend back. Some time later after some epic planning that would EPICALLY FAIL, Ferrus goes with other Primarchs to attempt to kick Fulgrim's and Horus's ass. But then they got totally fucked up by a bunch of reinforcements while Ferrus assrapes Fulgrim... until he becomes Slaanesh's boyfriend; and proceeds to cut Ferrus Manus's head off. Horus was pretty fucking pissed at this.
Despite this, Horus kept Ferrus's head, removing the remaining flesh and leaving it a bleached skull for his throne. Sometimes he would talk to it, lamenting that he had nothing but broken psychopaths instead of angels and strategists as his generals. Waging a galaxy wide heresy is a tough job.
In 40k?
There is speculation (in-universe and out) that Ferrus Manus somehow lives, and is hidden away on Mars by the Adeptus Mechanicus, though mentioning this to the Iron Hands gets them impressively angry, to the point that they might demonstrate how they think otherwise by using the speaker's own head. The marines who survived the Dropsite Massacre were in no doubt, having seen the visual evidence from multiple angles, but apparently the Iron Hands found the files too depressing and set about their hard drive with a bolter.
That being said, the Horus Heresy book series might currently be sowing the seeds of some sort of truth to the rumour... Fulgrim regretted not being able to convince Ferrus Manus into joining the side of the traitors, and so had Fabius Bile attempt to clone his brother with all of his memories intact so that Fulgrim might try again at turning Ferrus to Chaos. This did not work as the cloned Ferrus would become hostile at the suggestion of turning traitor and would have to be killed by daemon-Fulgrim.
That didn't stop Fulgrim from trying again and again, believing that Fabius cloning methods resulted in "imperfect" clones and so ordered him to try harder and create more, though fortunately, each one would get angry at the attempt to turn him away from the Emperor and end up getting beaten to death by his brother.
Also of note, the Ferrus Manus clones did not have the metallic arms of the original, this fact was hidden from the clones by making them wear gauntlets, which the clones found confusing but ended up rolling with, since their lives tended to be measured in hours.
The flesh is... strong?
Being the hardass flesh-haters that the Iron Hands are, one would suspect that Ferrus Manus might has well have been called Ferrus Clunis when it comes to the weakness of the flesh. In reality this is about as far from the distance as possible:
"They are not my hands. This fact is forgotten by my brothers - inexplicably, it has always seemed to me. The hands are strong, to be sure, and have created great things for us all, but they are not mine. And that counts for something. They forget that the silver on my arms comes from a beast that I vanquished. It is the mark of a great evil that I ended, and yet it persists within me... I would struggle to remove it now... I will not remove the silver from my flesh because I have learned to depend on it. The fault is with my mind. I rely on the augmentation given to me by my metal gauntlets, so much so that the flesh beneath them is now little more than a distant memory... A day will come when I will strip it from me, lest I lose the power to master myself forever. Already my Legion's warriors replace their shield hands with metal in my honour, and so they too are learning to doubt the natural strength of their bodies. They must be weaned off this practice before it becomes a mania for them. Hatred of what is natural, of what is human, is the first and greatest of the corruptions. So I record it here: when the time comes, I will strip my hands of their unnatural silver. I will instruct my Legion to recant their distrust of the flesh. I will turn then away from the gifts of the machine and bid them relearn the mysteries of flesh, bone and blood. When my father's Crusade is over, this shall be my sacred task. When the fighting is done, I shall cure my Legion and myself. For if fighting is all there is, if we may never pause to reflect on what such devotion to strength is doing to us, then our compulsion will only grow. Already I see the madness that path leads to, and so I shall excise the silver from my hands. In doing so I shall weaken myself and my sons, but nonetheless it must be done. The hands are strong, and have created great things, but they are not mine."
With what the Iron Hands do by turning themselves into cyborgs with only part of their brains being flesh this is as big of a FUCK YOU to the Primarch as possible. Or they thought that his doctrine was lacking on account of him receiving a viking crewcut courtesy of his once best friend and felt that they needed to protect themselves from such a fate. Either way, it puts the Iron Hands in an interesting light.
On The Tabletop
| Pts | WS | BS | S | T | W | I | A | Ld | Sv | |
|---|---|---|---|---|---|---|---|---|---|---|
| Ferrus Mannus: | 455 | 7 | 6 | 7 | 7 | 6 | 5 | 4 | 10 | 2+/3++ |
Ferrus Mannus is essentially God-King of Mars, functioning like a supreme Master Of The Forge Techmarine. In addition to all the fun godly powers given to all Primarchs, he also has Relentless and Smash, and straight up all ranged weapons fire at him at -1 strength value. At T7, that means boltguns, sluggas, and all manner of S4 shooty guns as are prevalent throughout all of 40k no longer have the chance of hurting him. Bringing him also makes your Legiones Astartes Iron Hands army function a lot like the 40k Chapter Tactics, in that all models gain Feel No Pain (6+) unless it already had one better.
He has Battlesmith and passes all repair rolls on a 3+ (That becomes a lovely 2+ thanks to his servo-arm). He also makes any of your vehicles with at least 1 AV of 13 or higher gain It Will Not Die. You are going to field tanks and Contemptor Dreadnoughts with this guy. He is good to them. His crazy 2+/3++ armor includes a Nuncio-Vox so Ferrus can help direct your heaping gobs of indirect tank fire, and it gives him a servo arm. It also lets him shoot two of his guns in the shooting phase, making him the shootiest primarch we currently have rules for, which include a Plasma Blaster, Graviton Gun, Grenade Harnass or Heavy Flamer.
Lastly, his hammer Forgebreaker (that one forged by Alucard Dorian Grey Fulgrim) is "typical" Primarch stuff in that it's Sx2 Ap1 Concussive Strikedown. Not bad, Fulgrim, you pretty boy fuckhead. You make a pretty decent hammer.
Considering his bonus Primarch powers are a force multiplier for a large tank army, and Legiones Astartes Iron Hands already gain several bonuses for tanks, you're bringing him with as many as you can field. Roll him alongside your tank columns so he can repair as you roll onward, knowing full well he can capably handle most any hand to hand unit thrown his way and is an excellent shot with powerful weapons in his own right. Just steer clear of Fulgrim. Just like in the fluff, he will fuck your shit up in close combat and make you a very sad panda.
And yes, the model's head is detachable.
| The Primarchs of the Space Marine Legions |
|---|
 Loyalist Corvus Corax - Ferrus Manus - Jaghatai Khan Leman Russ - Lion El'Jonson - Roboute Guilliman Rogal Dorn - Sanguinius - Vulkan |
 Traitor Alpharius/Omegon - Angron - Fulgrim Horus - Konrad Curze/Night Haunter - Lorgar Magnus the Red - Mortarion - Perturabo |