Zahan was a Nerazim intelligence agent who served in Vorazun's intelligence network. It was through him that Vorazun learned about troubles in Nerazim society and any tension that arose between them and the Aiur protoss.
When Taelus and a group of Nerazim insurgents took control of the Citadel, Zahan was able to tell Vorazun of the situation in spite of Hierarch Artanis attempting to keep it silent, though he could not find the identities of the attackers.
References
Burns, Matt. "Children of the Void." (Oct. 13, 2015). Blizzard Entertainment. StarCraft Lore: Children of the Void Accessed 2015-10-13.