Nightmare Reize takes the form of a giant power suit worn by Reize, looking both organic and mechanical and floating above ground. The suit is composed of a dark blue armor and a dark red sash similar to Reize's, covering dark flesh. The armor has a face in the center from which he can spit flames. His long arms are completed with large green claws. One hand is covered by a gauntlet going up to his shoulders with spikes, but he can produce fire from both. Reize's head can be seen on top of the armor, without visible pupils and surrounded by green flames.
After Specter Knight defeats the Enchantress, she calls Dark Reize to her to reveal his perfect self to the felon knight. Despite Specter Knight's attempts to change his mind, Reize agrees to let her change him. The Enchantress then floats Dark Reize into the air and sends pillars of green flames over his body, turning him into Nightmare Reize. The floor then collapses, sending Specter Knight onto rails that are used during the fight.
Nightmare Reize has three main attacks. He can:
- Shoot projectiles that can take on two main forms. The first are spinning wheels of energy, similar to those seen in the Enchantress and Dark Reize fights. The second are green flames shot towards the player that, upon hitting a rail, create slashable flames similar to those in the Lich Yard.
- Move to the right of the screen, avoiding attack, before charging to the left in an attempt to tackle the player. After the attack finishes, Nightmare Reize appears back on the right, presumably by some sort of dark magic or teleportation.
- Although not a direct attack by Nightmare Reize, when boss health is down to 2.5 points, the rails will disappear for a while. Specter Knight must dash slash through red lamps that get increasingly far apart to reach the single rail for the rest of the fight.
Nightmare Reize can only be damaged by attacking the head. All other parts of his body cannot be damaged. Any contact with Nightmare Reize or his attacks takes a full bubble of Will.
The battle takes place on two everlasting rails, with random red lamps. Specter Knight can only grind, which takes away some Curio use. The top rail is usually beneficial for attacking, but the bottom tends to have easier red lamp patterns.