These aren't masks, but skulls deformed by black magic. After all, this is very powerful and ancient undead; it has lost its humanity.
The unit will be on level seven.
Added 59 minutes later
I never paid attention to this artwork before. The Necromancer's staff is incorrect here; the game doesn't have a Namtaru figure at the end; instead, there are 2 spikes, and the other end has a simpler design.