Many, many moons ago I presented my players with a scenario in which they had to go about their business of pursuing the main storyline and, periodically, had to avoid a particularly bright and tough green dragon. The dragon was simply beyond them in terms of power and intelligence, but they eventually became arrogant and deluded themselves into thinking that they were clever enough to challenge the dragon (or at least pilfer some of its hoard).
I had a trustworthy ranger NPC/companion attempt to dissuade them from this activity by pointing out a smashed shield lodged in a tree not too distant from the dragon's lair that had formerly been of greater dweomer than any currently possessed by the party. Its former owner, a member of a powerful and ill-fated attempt to dislodge or kill Mr. Green, fed the dragon that day as did a number of others. The players heard him out, but decided to make the attempt anyway. Long story made short, they ended up fleeing with their tails tucked between their legs and down a party member or two. Nitwits. *chuckles & burps*
Provided that there are some clues or tales, would you mind having one or more encounters in the game that are either things that must be endured or avoided as no matter how clever, strong, magically potent, and/or stealthy your party might be, you don't have a snowball's chance in the Phlegethon of overcoming it in the game?
Humble pie: it's what's for dinner!