You let the dice fall as they will.

My players have been getting really, really cocky. I dropped hints about an ancient green dragon on a nearby mountain, and I planned for it to enter the story after they got to a much higher level than their current 9 and 10. Well, last session they called in some favors some high level mercenaries owed them, and decided to go kill the thing.

Upon arriving, it took one look at them, used its breath weapon, and dropped all of the party and all but one of the mercs to zero health. I rolled this damage openly and let them count it because I knew it was going to slaughter them. I wanted them to realize that there's a lot of stuff still in the world that could end them quite easily.

However, instead of just letting them die, I let them wake up back in town, missing a couple significant items. The one remaining merc bartered for the idiots' lives, giving the dragon some of their magical gear in exchange for letting him haul them back down the mountain.

This has achieved several things because I didn't pull punches during the combat. Now, they have a newfound respect for really nasty monsters, and are now playing less "walk in and smash" and are using their heads. They also now have reasons to go back to that dragon when they get stronger, both for revenge and to get their stuff back. Since they were also less than truthful about the size of the dragon, they've got a mercenary company that was a friend, now is hesitant to negative about hiring out to this party, and now they want to know why this random mercenary that they cannot find anymore was able to barter with a dragon. I could have killed them and let them just die, and that would have certainly created some interesting storylines for new characters, but there's something to be said for letting the dice fall where they will, and then learning a very valuable, but very harsh, lesson. I have wondered if there was something similar, or if we really would just see the death of Grog and a broken Vox Machina sneak back out of town in the case of a loss.

Full Show Spoilers ahead: Vox Machina has had some similar encounters. There's been times during the stream they've seemed like they were invincible. Then something comes along and strips the magic carpet of power, or kills a party member, and they step back and realize that there's some nasty stuff out there, and they are a tiny group of fish in a very big ocean. I think that the Kevdak fight would have been another of those moments. They'd come up with all these traps and plans that were nice on the surface, but they had no contingencies for when something went wrong. They improvised quite well in the situation, but it could have gone much worse because they didn't plan far enough.