Shadows. You could think of them as monsters, not unlike the ones we encountered here, save for the fact that they exist in a parallel reality that only a select few can access, and they're generally more cognizant.

[A soft snort of laughter follows.]

They reside within the collective unconsciousness, and thus many of them take on forms of gods and demons within various human mythologies. Anything from Jack Frost to Cerberus, though the latter lacks the three heads that Hades' hound is typically known for.

One of the monsters in the first attacks did look identical to a Shadow called Pisaca, however.