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I also looked into what is known about Voidic creatures in general, and learned the following:
Most evidence points to most creatures of void having no body, so most creatures of void would not be affected by weapons (besides spellstriking) or several spells that are meaningless without a body (such as Root or Wither).
There are 2 exceptions to this:
Void is strange and paradoxical, so some things do have bodies, or behave in ways that no-one understands. Some incorporeal creatures of Void gain bodies when they are near the Bulwark, in Vigor, as a side effect of the Bulwark itself.
I also found some basic evidence that some, though not all, creatures of void have strengths and weakness similar to undead. However, it's unclear if they can be turned by a Mystic.
Furthermore, information on exactly what is in Void is scant, because so few people survive to report on it, and many of those that do have neither the sanity nor the words to describe it.