Deep Nightmares look much like the regular Nightmare, resembling humanoid chameleons with thick forearms and have purple smoke emitting from their back, but they have some distinguished traits as well. They are slightly taller, their horns and tails are larger and more jagged, they have several more teeth with tusks in the corner of their mouth, and they have larger and more spikes on their back. Also, their skin is coloured a darker blue and their marking glow in a darker electric crimson.
The Deep Nightmares are able to move around by teleporting into an area in a puff of purple smoke, or emerging from black puddles on the ground. As a Dark Gaia Minion, the Deep Nightmares can possess people by latching onto the deepest, darkest part of the heart, and generate despair, negative emotions or even erratic behaviour that are completely opposite to the person's personality.
Being an evolved form of the Nightmare, the Deep Nightmare has strength, speed, durability and intelligence above the regular Nightmare. They possess elastic bodies, allowing them to stretch their limbs over a slightly larger area. They can even enlarge their limbs and amplify the power of their physical strikes with dark energies, though doing so exhaust them and leave them weak and invulnerable afterwards for a few moments.
The Deep Nightmares have a certain degree of skills and cunning in terms of hand-to-hand, being able to fighting opponents with combos and guard against attacks with their arms. They can also use the spines on their backs as melee weapons.