M.H.
My kids - 13, 10, 6 - all went through that stage, and the youngest still does on occasion. He's even scared to use the bathroom with the door closed, but only when he is in that "scared" phase. It last a week or 2. Then he's perfectly fine for several months, then it hits again. This is a normal part of development. They begin to recognize between 4-6 that they are mortal, and that bad things happen. Then their creative little brains take that to the extreme. Just find something that works for you. I find that if we have a particularly bad night, and I have taken him back to his room several times, it is ok to let him sleep on our floor or in our bed. The next night he must sleep in his own bed, so it doesn't become a habit.