the stuff of nightmares

1. That which is especially disturbing, grotesque, or horrific. My grandparents' cellar was always the stuff of nightmares when I was a kid—dark, dirty, and positively crawling with spiders. What we saw when we knocked down the rotten wall was the stuff of nightmares, as dozens upon dozens of rats came pouring out.

2. That which is unbearably unpleasant or distressing. The things that disease does to your body is the stuff of nightmares. Having to endure the death of your children—it's the stuff of nightmares. I can't stand to even imagine it.