the stuff of which nightmares are made

1. That which is especially disturbing, grotesque, or horrific. My grandparents' cellar was always the stuff of which nightmares were made 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 which nightmares are made—a huge, writhing mass of rats.

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