The Unspeaking was a Chapter of the Word Bearers Traitor Legion that took part in the notorious Calth Atrocity during the opening years of the Horus Heresy in the early 31st Millennium. This Chapter was under the command of the so-called "Crimson Apostle" Zardu Layak and his Ashen Circle destroyer units, who were charged with enacting some of the worst civilian massacres of the opening hours of the battle.
Both the Unspeaking and the Graven Star Chapters were known to have largely retreated from the planet's surface during the final hours of the war, and would go on to further infamy in the Shadow Crusade.
The Chapter of the Word Bearers Legion that came to be known as the Unspeaking was once known as the Ochre Gate. Its fate was tied to its master, the Dark Apostle known as Zardu Layak, a former officer of the Ashen Circle. A devout destroyer of all that pertained to false superstition and errant creed, he oversaw the burning of a hundred temples and countless tomes of lies in the Emperor's service.
But following the Word Bearers' brutal censure on Monarchia at the hands of the Emperor, Layak became soulsick and lost. Following Primarch Lorgar's pilgrimage into the Eye of Terror to commune with the Primordial Annihilator, Layak fervently converted to Erebus' and Lorgar's new teachings.
But even this did not fill the terrible void within him. So it was that when he began to receive visions in which he believed his Primarch spoke to him directly, he kept it concealed from others. These visions instructed him to reconsider his work, not to burn the books and lore he encountered, but to read them, covet them, to search in them for the hidden truths that that the Emperor had not wished His sons to know.
Soon the former Ashen Circle warrior named himself from the Book of Lorgar as Zardu Layak, "Eater of Wisdom," but who came to be known more swiftly by his Legion as the Crimson Apostle for his fiery raiment and the scalding blood in which he daubed his armour.
Assigned to the Chapter of the Ochre Gate as its Dark Apostle by Erebus, who sought to channel his fervour, Zardu Layak quickly proved more than a mere voice of the First Chaplain. Soon the Chapter began to seek missions of its own, away from its parent Legion for long stretches, and each time it returned it was further changed.
Like a cancer, Zardu Layak had embedded himself deeper into its carcass, so that after a time, the Ochre Gate was no more and now the Chapter of the Unspeaking stood in its place, with Zardu Layak its de facto master.
Though Kor Phaeron was disquieted by the Crimson Apostle's growing power, much of which he would keep hidden until the battle on Calth, Lorgar pronounced himself pleased by Zardu Layak and his devotion to the word of the Primordial Truth, assigning the Unspeaking and their master to command a key assault at Calth during the Battle of Ithraca, though so changed had they become by their devotion to Chaos and the "gifts" of the Warp that complete concealment was needed for them until the hour came to strike.
- The Horus Heresy - Book Five: Tempest (Forge World Series) by Alan Bligh, pp. 61, 105, 107, 109, 111, 250-251