Ferrus Manus

"'You are weeeeaaaaaak, Cooooorvuuuuuuuuuus....'"Ferrus Manus was the Primarch of the Iron Hands, and tenth son of the Emperor.

Official Canon
Ferrus Manus was found on Medusa, and was accepted as a honorary member amongst all of medusas barbaric clans. He earned his iron hands when he shoved a silver dragon into lava, which ended up coating his arms with the creatures metal, making them hard enough to act as living armor. When the Emperor found him, he was given command of the Iron Hands legion.

When Ferrus Manus first met Fulgrim, the two became fast friends after a weapon smithing competition. However, after dismissing Sanguinius' gifts of art, Fulgrim gave Ferrus the epithet of "The Gorgon," which Ferrus took a liking to.

Ferrus Manus was unhappy with the obsession his men took with replacing their flesh with augmentations. Believing that his iron flesh set a bad example, he wished for the day that he could shed his unnatural condition so that his men would trust in their own innate abilities.

Horus Heresy
Unfortunately, this friendship would not stop Fulgrim from beheading Ferrus Manus at the Drop Site Massacre, when the loyalists who were sent to stop Horus were suddenly betrayed by their fellow legions.

While Ferrus himself is dead, Fulgrim tried many times to have him cloned and to try to convince him to join Chaos. Each attempt was unsuccessful.

TTS Canon
Though he is deceased, Ferrus Manus continues to haunt the mentally unstable Vulkan in the form of a ghost that persistently berates him. The same ghost haunts Corvus Corax when Vulkan died temporarily. He is refered to as a "skull ghost", or "skost."

Both Vulkan and Corax are very annoyed by the ghost's presence, and consider it to be a poor imitation of their brother. Whereas the ghost will say things such as "The flesh is weeeeeaaaaaaak...." Vulkan said that the real Ferrus would not say such a thing.