Thinking of nightmares, it just reminded me of one i had a year or two ago.
I was on some sort of station (i think in space, maybe a moon) and it was pitch black. I was unarmed, and was creeping through the station.
I notice a presence, there was something else there with me, and i knew it wasn't human, so i took every step with extreme caution to make sure i didn't make a noise. I could hear the creature making sounds, like a hissing buggish growl. And i was afraid the thing would jump on me.
I knew I had to kill it, otherwise it would kill me, I mean cmon, pitch black station, inhuman creature hissing in the dark? It needs to die. So I miraculously find some blunt object I can use ( I think it was a broom, or maybe a baseball bat?) until i could find a better weapon, like a gun or katana. and tread into this big room, with a stairwell in the center that went up to an open rail floor (kind of like a second floor with a balcony view to the floor below)
i crouch walk up the staircase slowly, the room is Illuminated with a very dark blue hue, as if the light is barely powered, I can still hear the creature, and im scared its at the top of the stairs. I keep moving, get to the top, and see a doorway to my left, i get close to the door, and what do i see?
I see the Alien from the Alien movie series. I freeze up on the spot, thinking it noticed me, but it didnt. Instead, it makes a growl-like sound, and walks through the doorway out of view...
I follow it at the same pace, intent on killing it with its back turned, and wake up at the threshold of the door...
sounds like it WOULD be a plot from Alien doesn't it? But I've never watched any of the movies ><
EDIT: Probably not my worst nightmare, but I don't tend to remember them, as they are meant to be forgotten :P