With inputs from Marius in discussing what they noted in the battle, this is what Zauberei comes up with (and assumed to share with the party)...

This was not undead, not exactly, but sort of. This was an animated object ... a type of construct ... but one that has a weakness some magically animated constructs do not, in that this particular construct was haunted (haunted, but not a "haunt," per se). As such, attacks are more effective against undead ... such as Hargrimm's banelight vortices.

That said, the undead energies were granted the durability of a construct, but was a mindless spirit. Constructs, as a rule tend to be unhindered by morality in combat so are surprisingly effective for their relative "weight class," and are moderately durable. Animated objects, in particular, are hard objects, and thus resist weaponry, like they did with Razvans bullet.

That's the best guess of "what" it was, but who knows "why" it was, until you look at the block of stone with the dark stains on it, and realize that this is where prisoners of Harrowstone were likely once beheaded.