I've just noticed something about the Zahhaks, Equius, Horuss and Darkleer; all of them have glasses that obstruct their eyes. Equius wears black glasses, Horuss wears black googles and Darkleer wears black glasses connected to his helmet. Does that relate to their Void aspect?

Yep, Shades literally obscure one’s sight, there’s two ticks for Void there, obscuring, and also preventing Vision (Which is Light-bound). Shades are Void-y enough to actually mask the whispers of the Horrorterrors.