It could something more subtle, like the Order Corruption.
In the Emerald Nightmare:
- "Five keys to open our way. Five torches to light our path." (He's talking about the 5 sigils Zovaal needed to open the gate to Zereth Mortis)
- "Her heart is a crater, and we have filled it." (Sylvanas being manipulated with Zovaal lies)
- "From the earth, he draws strength. Our earth. Our strength." (Zovaal absorbing the power of Azeroth)
In Ny'alotha:
- "Before the last shadow falls, the father of sleep shall savor his feast." (Mueh'zala devouring the souls of all the dead trolls thinking that he wont need them anymore, because Zovaal would get him a lot more souls)
- "Five lanterns now darkened. The flames they seek will light the Masters' way." (Again talking about the hiden sigils that Zovaal need to open Zereth Mortis gate)
- "The cunning ones kneel before six masters, but serve only one." (The Nathrezim faking allegiance to all the Forces, but actually serving only Zovaal)
Add too how he looks:
(He looks looks really diferent from other Void spawns. more like a really corrupted and deformed Nathrezim. Plus, his name feels more like a demon/Nathrezim origin than the typical names of Void minions)
It's not confirmed, but all those things together strongly hint at him being the infiltrator in the Void ranks.