About Nybeth Obdilord

[Cleretic]

Since the ghost cameos in PotD are typically of Etheirys, I would think Nybeth also originates from this world, rather than being an interdimensional crossover.
It's likely that, like Final Fantasy Tactics, events very similar to Tactics Ogre took place at some point in history in the "Matsuno region".

Theundead mobs in PotD are explicitly zombies, some of their names describe the decayed nature of their bodies (Bloated Archer, Jaundiced Tribunus, etc). Nybeth's Echo flashback with Edda also makes it pretty clear that he's dealing with bodies. The way they behave is also a perfect match for the undead Nybeth was working on in Tactics Ogre; in that game, he worked on trying to revive a person's body and soul at once, and the results of his experiments are actually what we see in PotD: well-preserved and physically functional mindless zombies, that only reunite with their soul in their final moments before re-death. In both games that manifests as them becoming 'themselves' again for their death line and that's all.

PotD does have ghost mobs, but they're just sorta there on their own thing, I think.