"As you pull the blade free, it begins to rapidly rust away. The legend was never the blade itself, it was the strength of the heroes who led the way.
That strength has caused the ground to shift, rocks fall, everyone dies."
I want to steal this idea.
Imagine the moment the stone shatters when you are finally deemed worthy
“Noooooooooooooo!”
“Fool, i can only be defeated by having the blade stabbed through my heart! As you are unworthy, you cannot kill me!”
a single crack loudly appears in the stone, the villain stares in shock, after a suspenseful pause the stone starts rapidly cracking further apart until it all falls off to reveal a gleaming flawless blade
“No! How can this be?! Have your journeys made you worthy?”
villain is stabbed to death yada yada yada… after the protagonist gathers themself the blade suddenly sinks into the ground and they pull it out with a chunk at the end again. The power behind the blade starts talking to the protagonist
“Yeah no sorry, you’re still not worthy, but eh fuck it it’s been 500 years and that shithead needed to die, sorry to dissapoint. Please leave the sword somewhere accessible when you’re done with it yeah?”
… Dude, that’s over a 20% chance of success. You sure you dont want to rethink this? it isnt even as improbable as some saves in normal play.
13.1% if BI is a d6, or 20.7% if they’re past level 5 and it’s a d8. Certainly doable