This powerful boss resembles a gelatinous pile of stinking ooze littered with bones and skulls. In its second form, it has a hard segmented shell across its back, making it resemble some kind of bizarre snail. The shell protects its weak spot: the core.
The creature was created by Mundus out of inorganic substances, and it is unknown whether it is even conscious. It is powered by a core which it rarely exposes, and which can absorb Devil Trigger power. If it is allowed to absorb enough, it can activate its own Devil Trigger. It also possesses a multitude of offensive mechanisms, and will change its attack pattern each time one of its cores is destroyed. When in its gel-like form, it can swallow enemies and teleport them to an evil dimension in which their trauma manifests as evil spirits. In order to be damaged, it must be forced into a more solid form by activating embossed discs on the ground, restraining tools used to keep it from overriding. However, in this form it will encase itself in a hard armor, and attack even more viciously.
Dante first encounters Nightmare within the castle cathedral, but he forces it to retreat. It later appears while Dante performs the ritual to transform the Philosopher's Egg into the Elixir, and is again forced to retreat. After Dante enters the Underworld, he is lured into Nightmare's den by Trish, who is pretending to be injured. However, Nightmare attacks as soon as Dante enters the room, and while fighting it, Dante is also attacked by Trish. Dante manages to finally destroy the Nightmare, and coldly leaves Trish in the room while he goes on to Mundus.