I have a question: why the volcra don’t get out of the fold borders at night (I’m guessing that they can only exist in complete darkness)? If I’m right, during an eclipse there are a complete darkness, so they should come out, right?

There is the matter of starlight/moonlight, but the real issue is that the darkness of the Fold is unnatural. It isn’t ordinary darkness. The volcra were created by the same power and the Shadow Fold is their only real home.