This was a fun one. I entered a 500 word science fiction story contest with the entry below. I didn’t win but it was a good exercise in forcing yourself to be concise.
He could feel something wet leaving his body and dripping onto the ground. Sweat he hoped. Blood more likely. The Arm stopped above him. He held his breath and lay motionless. The wall creaked and a faint light appeared overhead. The Arm paused, sensed for movement one last time, and then disappeared as the light faded out behind it.