After all they've been through, our heroes have defeated the baddie. Time to celebrate, right? Unfortunately, the villain's pad hasn't been renovated in centuries, and for some odd reason, killing off the bad guy resonates throughout the entire structure. No time to party. They have to run for it, before the ceiling crashes on their heads!
The Collapsing Lair - The demise of a villain's base of operations after their defeat, an event that results after accomplishing a goal in a structurally unsound place (usually an ancient tomb or treasure room). Often used as a grand finale or as an easy way to obliterate any remaining minions or evidence of the villain's existence.
On rare occasions, the villain may be alive while the place falls down, and ends up "going down with the ship," so to speak. Having a herd of unicorns trampling by your front door is certainly of no help.
I hope Haggard's insurance covers unicorn stampedes.
At other times, a Collapsing Lair may be caused by a booby trap or a switch, meant to activate a snare that will bury any who trespasses.
Run, Indy, run!
Now hold on a second - how come all these ancient structures get to have all the fun? Why not shake things up a little by having the lair be a starship, collapsing in a great ball of fiery glory or something?
Oh, right ... they've done that ...
This is a fun trope to apply if there is a sound reason for it, otherwise it looks and reads tacky when it's thrown in without foreshadowing it, or it's there for no other reason than to pour salt into the wound.
And thanks for visiting my laire; mind your heads on the way out ...
What are your favorite Collapsing Lair scenes? Which ones made you groan? Have you ever thought of using this trope, or have you?
I'm David, and goodbye, Beni ...