Nightmare

Nightmare is another kind of hologram that can roam the Arena (other than White Widow), and happens to be THE biggest threat you can encounter in it. He is always shrouded in darkness, and thus it's hard to make out any of his monstrous features. He's first encountered on Night 5, as a final boss of sorts.

As mentioned in the in-game notes, he was inspired by the legendary demons seen in numerous cultures and religions, and also by the various nightmares the creators had in their childhood.

He is the only robot to be implied to have a level of sentience, rather than mindlessly following his programming. This is evidenced by him directly talking to the player, and approaching them outside the Arena.

Behavior
The most recognizable trait of his is that he causes all lights near him to turn off. Therefore, if you want to know where he is, look for places where there's unexpected darkness.

Unlike any other robot or hologram, his movement is neither a predetermined pattern nor a randomized wandering about. Instead, he always goes straight after you, regardless of whether he can see you or not, only stopping briefly every few steps to give you SOME chance of survival. As you can probably imagine, this means that you cannot stop anywhere for too long, or else he'll get you.

Like most other robots, he will start chasing you the moment he sees you. If he manages to get close enough to you, the resulting radiation will be an almost certain instant kill.

He never appears in the Arena before 1 AM (specifically, 20 seconds after the clock shows 1 AM).

Sounds
When idle, he emanates an odd, staticky noise. As mentioned within the in-game notes, to s ome people it sounded like wind... while others said that it reminded them of a vacuum cleaner.

When chasing you, he emanates a loud sound of static.

Suggested tactics

 * If he gets near a closed door, he will immediately teleport somewhere else. However, it might be necessary to wait until he starts moving before shutting the door closest to him, as he might not be standing close enough for it to affect him. Sometimes, your best bet might be to purposefully expose yourself to him and THEN close the door. This issue does not apply to the Ghost Challenge, since the doors have a wider range of effect in it.
 * By positioning yourself correctly, you can trick him into entering certain hallways. Just stand right behind the corner, and/or right behind a doorway (while pressing yourself against a wall in the latter case), and wait until he makes his move. Once you see that he entered the hallway you wanted to lead him to, carefully make your way out of there without exposing yourself to him. Doing so will buy you some precious time, as it will likely take him some time to get out of that spot.
 * It seems that the other robots are terrified of him, and will immediately get away from him as soon as he gets close. It's strongly recommended to use it to your advantage, especially during the Ghost Challenge.