Night Guards are spirits of unique nature that are immune to all kinds of nightmares due to the fact that they were created from these said nightmares.
Night Guards, like any other spirit in the Nightmare Realm, are immortal. They can't age, can't get old, but at the same time they are able to get killed by their own kind and get injured. Their wounds do not regenerate - they all heal naturally.
A nightmare, from which a Night Guard was created, can affect either the entire being of a Night Guard, or how their powers feel like to a human. For example, a Night Guard created from the suffocation nightmare would make the humans that they use their powers on feel like they're being suffocated, but these humans won't die because of it unless the Night Guard truly wants it.
Most of the time the Night Guards use their powers to pick the stray Drowned humans off the streets of Nighthawk. If a human does not come willingly, the Night Guards use their powers on them, turning them into a trance state. The side effects of that can be different, depending on a Night Guard that did it, either their origin nightmare or personality. For example, an extremely violent Night Guard’s trance spell can make a Drowned human feel immense pain.