Nice Job Breaking It, Hero!

The hero has accomplished their goal; they've killed the Big Bad, or defeated them forever, or at least scored a major blow against the antagonist. They've done what they set out to do, but not so fast! It turns out that by the very act of success, they've unwittingly made things worse. Maybe the now-dead antagonist was actually holding back an even greater evil.

Maybe the villain, having been defeated or damaged, is now transformed into a new, ultimate, unstoppable, invincible, angry form, generally against the villain's will (or against their expectations, anyway). Maybe the villain, despite their villainy, was serving some other greater good, keeping the world/universe/nature/whatever politically or literally balanced and their demise throws things into chaos.

Maybe that princess you saved is a cold-hearted tyrant at the head of an oppressive regime.

Perhaps taking down the villain has resulted in an Evil Power Vacuum, and now even worse guys are fighting to fill the void that the previous villain left, without a single regard for who else gets hurt or perhaps the only means of foiling the villain involves questionably massive "collateral damage".

For whatever reason, the hero's victory over death and destruction directly or indirectly leads to an even greater wave of death and destruction, or at least an even greater threat of such. Oh dear.

Maybe the villain themselves will warn the hero about the possible consequences, as a last-ditch attempt to save their own skin, or as a bitter "parting shot" to ruin the hero's victory ("You fools... do you even know what you've done?").

Villains being villains, this may just be a bluff. Or maybe the villain will just lament over how the hero defeated them despite all their efforts - it wasn't some sort of plan of theirs for the hero to "win" and thus make things worse (if it was, that'd be My Death Is Just the Beginning).

Then again, Poor Communication Kills often comes into play as the villain will usually be uselessly vague and give no concrete reason for why foiling him would be bad. On the other other hand, a clear warning might be taken as Cassandra Truth anyway. You can't win.

If the mission was a rescue, it was an Unwanted Rescue and the former captive may bitterly inform them of the true facts.

In most cases, of course, the hero's new mission is to stop the new danger they've unleashed, preferably in a way that doesn't spawn ever greater menaces.

Darker plotlines may end the story right there instead, and it's always possible the villain's demise inadvertently resulted in the irreversible destruction or horrifying mutation of the world.

If used excessively, this trope often leaves an audience with a sense that the hero shouldn't have tried to change things for the better and indeed shouldn't have even left home, which can be a Family-Unfriendly Aesop, but also a more literal moral about getting seriously involved in things without learning much about them.

This trope can be annoying in video games as often-times Stupidity Is the Only Option, particularly in the case of a MacGuffin Delivery Service... It is not particularly fair to try to make the player feel guilty about a course of action they had no control over, and indeed, might've gotten a "Game Over" if they attempted to not fulfill the objectives in question.