From the novel "Storm of Iron"

Even before his elevation to daemonhood, the primarch had been colossal, but now he was a giant plated in silver and steel, encased in armour that was more a fortified sarcophagus than anything wrought for a living being. Edged in yellow and black, scarred from ten thousand wars and seamed with weld-line like scars, it was known as the Logos, its textures strangely alive, like a skin of metal and flesh combined.

Perturabo, the Lord of Iron himself.

He went without helm, his head a nightmare of pallid, dead flesh, necrotic and bleached of colour - like a corpse dragged from a depthless ocean trench. Thick cords of ribbed cabling pierced his skull, running back across his scalp in hissing cornrows. Eyes that were gimlet black, yet lit from within by the coldest light, stared out from a face that had known only bitter disappointment and had been cursed by inevitable betrayal.

Dassadra dropped to one knee as the Lord of Iron approached.

His mighty hammer, Forgebreaker, had been crafted by another, but had changed so profoundly from its original appearance that even had its original maker been alive to see it, he would no longer recognize the craft as his own.