You can find information on Zardu Layak in Book 5: Tempest, as well as Slaves to Darkness, where he is a major POV character. I'd heartily recommend both, especially the latter.
To answer your questions, his title is "First Chaplain of the Unspeaking". He's essentially a Dark Apostle and a Sorcerer combined, and his rules reflect this as he provides both psychic support and the zealot special rule.
On his relations with Erebus and Kor Phaeron:
Assigned to the Chapter of the Ochre Gate as its 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 atrocity on Calth, Lorgar pronounced himself pleased by Zardu Layak and his devotion ot the word of the Primordial Truth, assigning the Unspeaking and their master to command a key assault at Calth, though so changed had they become that complete concealment was needed for them until the hour came to strike.
At the start of Slaves to Darkness, Zardu Layak is Lorgar's current "favourite". But as of the end, he
