Nightmare creatures are actually a special type of two-dimensional lifeforms.
Two-dimensional lifeforms are fundamentally different from normal life.
For example, a monster drawn on a child's doodle, in the eyes of normal people, belongs to part of the drawing. From a perspective outside the drawing, one merely needs to tear up the paper, and the creatures within it will be destroyed along with it.
But to the heroes within the drawing, or to those who live in the doodle world, the monster is a very real Demon King, and as long as the paper isn't destroyed, it can use the pattern on the paper as an "anchor" to come to the real world time and again...
If one were to cross the boundary between three and two dimensions, bringing life to this two-dimensional monster, then it becomes a special two-dimensional lifeform, a mysterious being that possesses both immense intrigue and extreme fragility at the same time.