"I have heard tales of you, though we never met. It is an honor."
Zayl is a Necromancer. He carries the undead skull of Humbart Wessel in his pouch for company and for aid in times of need, along with an ivory dagger crafted from Trag'Oul sealed to Zayl's soul. Zayl would later go on to become one of the first of Tyrael's reformed Horadrim.
The Kingdom of Shadow
Zayl was first introduced in "The Kingdom of Shadow" as a support role or secondary character. Though there is a significant amount of character development throughout the novel/book, he is often seen from the point of view of the protagonist.
Moon of the Spider
In Moon of the Spider, Zayl is the main character, and much more information about his character is released from his own perspective. Zayl does not seem to have a personal connection to Trag'Oul, as Mendeln in the Sin War trilogy. He is also shown to have a compassionate/emotional side behind his cold exterior.
Storm of Light
After Tyrael took on the mantle of the Aspect of Wisdom, he soon discovered the High Heavens were being quickly influenced by the corruptive essence of the Black Soulstone. With his fellows archangels standing against him, Tyrael sought to hide the Black Soulstone on Sanctuary. To this end he went about reforming the Horadrim, selecting various mages, warriors and scholars for the task of stealing the Soulstone and hiding it from both Heaven and Hell. Zayl was one of the first of this renewed Horadric Order, and would provide critical support in the raid on Heaven. Not only would he forge a satchel strong enough to contain the corruptive influence of the Black Soulstone, but he also awakened his hidden nephalem powers and saved fellow Horadrim, Jacob of Staalbreak, though Zayl would lose an arm in the process. After the heroic efforts of the Horadrim, Tyrael relieved Zayl and the others so they might recuperate from their injuries and the influences of the Black Soulstone.
Reaper of Souls
"I am Zayl, a necromancer who seeks to maintain the balance."
Zayl entered the Blood Marsh after he sensed a deep imbalance within the area. He came to heal the land. He was encountered by the Nephalem, as the Reapers launched their assault on Sanctuary. Zayl told the Nephalem about how he helped steal the Black Soulstone from Heaven, but wasn't in the Tomb of Rakkis when Malthael came to obtain the artifact. Humbart Wessel stated that Malthael wouldn't have succeeded if Zayl was there, but Zayl just told him to be quiet. He told the Nephalem of his goals, and they told him of their own goal to slay Adria. He told the Nephalem that if she was slain, it would take time for equilibrium to be restored.
Zayl appears as an NPC randomly in the Blood Marsh or Path of the Drowned in Act V of Diablo III. He usually has only one dialogue option, "The Bog," but has another option when playing as a Wizard with Lorath Nahr as a companion. His arm appears intact, despite the loss of it in Storm of Light.