The Nightmare Phenomenon
The Nightmare takes the form of an artificial entity with a small torso and a head with an red eye, and arms who resemble a DNA helix. It will seek any reploids nearby and possess them. When destroyed, they leave behind a blue orb called Nightmare Soul, which will make the Nightmare resurrect if not collected. When affecting an environment, it will make strange objects appear, as well as calling forth unnatural phenomena such as blizzards and acid rains.
Aside from the common Nightmare, there are also the Zero Nightmare, who is a Zero replica made from the Nightmare Phenomenon which possesses the same abilities as the original Zero, and the Nightmare Mother, who is an humongous Nightmare which resembles a protein, controlled by a small mechanic eye.