Appearing as a boss in 2001's Devil May Cry, Nightmare is a demonic bio-weapon that attacks Dante on mutliple occasions, each time more powerful, it can only be damaged by attacking its life orbs. It sports two forms, the first one being a pool of poisonous sludge that tries to trap Dante in a prison where Dante must defeat a boss from earlier in the game to escape.
Its second form which Nightmare is magically forced into is a physical form that uses spike attacks which either attack Dante in a stabbing motion or attacking him like a boomerang. It can also drain Dante's Devil Trigger gauge in order to use an ice beam attack. This form is when its life orbs appears. Dante eventually kills Nightmare.
In Devil May Cry 5, the character V can spend his Devil Trigger to summon a giant bipedal demon also called Nightmare. This Nightmare can also charge up a beam attack but its relation to the original Nightmare pre-release is unknown.