Deimos

He is an Alieniues from Nightmaruius.

Appearance
He doesn't really have a natural form. He looks like your worst nightmare. (e.g. if your afraid of clowns, he would look like an evil clown) He can even turn into a whole illusion (e.g if your afraid of the dark, you would be in a room with no light at all).

Powers
Turning into your worst nightmare to scare you into submission.

Note
Feel free to add a picture of your worst nightmare. :]