Nightmare Xisenin is, for the most part, completely identical to Xisenin in shape, but he has the grey skin of a Nightmare Irken and serene blue eyes.
Nightmare Xisenin can talk, but he never does and nobody knows why. He has a constant smile on his face that can never be wavered by insults or distractions or even failing in a task. When hunting, he uses fear as his weapon. He never communicates with anyone through speech or writing, only body gestures. He is known for his undying persistence in a mission, and will stalk someone for years until they break. He rarely ever walks and usually only moves through light warping.
Nightmare Xisenin has the ability to "light warp", the opposite of a shadow warp, in which he can teleport by simply walking into and out of any light. Nightmare Xisnenin can also hide inside people without them knowing. He cannot control people directly while he's inside them, but he can interact with every system in the person's body, such as make person feel hungry or tired. He can also change shape into a mass of shadowy tentacles, but that's the only thing he can change shape into. However, the number of tentacles and where each tentacle starts can be changed, so he could be one tentacle or a thousand. He lacks telekinesis but can create shadowy tentacles that are connected to him and uses them in the same way. Unlike Xisenin, Nightmare Xisenin can use lights not only as walk-in portals, but he can also teleport directly to a light without having to walk into one. Nightmare Xisenin is harmful to most forms of technology and all recording devices. The mere presence of him causes cameras to malfunction and eventually show nothing but static. Nightmare Xisenin has no choice for this, he cannot voluntarily allow a camera to work.