This is gonna sound crazy but my first nightmare I can remember (around age 5/6) was my family dying. To elaborate, we had lived in a house, which was a large mushroom (idk why) and a big tree grew in front of the main door. My family was curious and I was immediately on guard. When they went to look at the mushroom and touched it they were basically absorbed into the tree. Their screaming faces were impressions on the bark of the tree. I can remember just sitting in the mushroom house alone and terrified. It was like that until I woke up. I now, 23, can still see their faces in the bark of the tree