the stuff of (something)

That which is utterly characteristic of a particular kind of thing (which is usually pluralized). My grandparents' cellar was always the stuff of nightmares when I was a kid—dark, dirty, and positively crawling with spiders. I've been playing table tennis since I was a child, so actually making it to the world championships is the stuff of dreams for me. The team made an absolutely incredible comeback, overcoming a 20-point deficit in the final minutes of the game. I tell you, it was the stuff of legend.