A Nightmare-type aberrant is characterized by being able to alter the subjective experiences of those around it, and also seems to be one of the two types where the Red Guard has to eliminate entire villages to stop, which seems to indicate that they can often spread through a population and infect them in some way.
When Lacer responds to the Raven Queen’s letter, his first thoughts about seals like that apply to limiting of someone’s subjective experiences, isolating memories or senses, or ability to wake from sleep, etc
Interestingly, these mostly seems to be the mechanisms through which you can limit a Nightmare-type.
I feel like I have more indications of this, but I’m too sleepy, so I’ll come back another time with those.