Ferrus Manus
Ferrus Manus, literally meaning "Hands of Iron", was one of the twenty Primarchs created by the Emperor before the Great Crusade. Ferrus was stolen away by the forces of Chaos just as his brothers, and spent his first years on the planet Medusa.
Ferrus Manus' life
Medusa was a harsh, unwelcoming planet inhabitated by a tough and stoic people. Ferrus would not join these tribes, as many of his brothers did. Rather would he seek out physical challenges, to make himself stronger and more resilient. These tests ended with his battle against Asirnoth, the Great Silver Wyrm, a metallic beast impervious to harm Ferrus had to drown in magma to kill. The melted flesh of the worm fused into Ferrus' flesh, giving him the true metal hands his Legion would take its name from.
Ferrus now returned to the tribes and started lecturing them on technology. The clans advanced at a stunning pace and Medusa prospered. But Ferrus took care to leave the inter-clan rivalry and fighting by never taking sides, as he saw the competition between them as healthy and good.
The Emperor eventually came to Medusa, and Ferrus Manus tested himself on him as well, in a cataclysmic battle that is said to have laid waste to entire mountains. Finally having found someone equal Ferrus accepted the Emperor as his master and took command of a legion of Space Marines.
Thousands of planets would fall for the remorseless Iron Hands under Ferrus' leadership. Each weak link in humanity was to be purged if mankind was to grow stronger. Scholars spoke of the wave of piety and faith that spread before of the Iron Warriors; with many planets suddenly motivated to look into themselves when they heard that the Iron Warriors were approaching.
Rumours has it that Horus approached Ferrus Manus before the heresy and tried to lure him to join Chaos. Ferrus Manus was so enraged at this proposal that he attacked Horus and nearly killed him. Horus managed to teleport back to his own battle barge and escape, although barely.
The Iron Hands was one of the loyal legions the Emperor called to Istvaan V. Given the large distance Ferrus Manus took the Legion's fastest ships and departed only with his veteran chapter. They were all lost during the battle, struck down by the traitor legions.
Some say Ferrus survived Istvaan and now resides on Mars, but the Iron Hands strongly refuse this.
Controversy around the Necrons and Ferrus
Some players have linked the Asirnoth, the Great Silver Wyrm to the Necrons, more exactly stating that Asirnoth was the C'tan the Dragon. The artifacts of old said to be located on Medusa combined with the "living metal" that fused into Ferrus' hands have been pointed out, and has been said to have given Ferrus' the ideas and technology he learned to the tribes afterwards. It has been stated, however, that the new Necron codex was still largely unwritten at the time the Iron Hands-article was published, and that the concept of the C'Tans were developed later.