This a story of two similar situations on opposite ends of the success/ failure spectrum, separated by a few sessions of play.
Our rogue had in his possession a dagger in which was imprisoned an elder demonic entity. They (the rogue and the demon) had b made a deal that if the rogue could trap a hundred souls in the dagger for the demon to feed on, then the demon would use that power to free himself and serve the rogue.
We entered the tomb several levels ahead of our dm's intended schedule and proceeded to successfully make our way through the individual lesser challenges and into the final chamber of the tomb… where we were promptly faced by an ancient five-headed hydra of multiple elemental types.
While the rest of the party provided distraction, the rogue decided to fling his demon dagger into the crook between two of the hydra's necks; natural twenty, followed by another to confirm. Dm rolls saves for the hydra. Double ones. The hydra crumbles apart as its soul is devoured by the demon, who agrees to count the creature as fifty souls.
A few sessions later, the party encounters an ancient red dragon of colossal size which our dm had dubbed the Gygax-killer. We as a group figure that our earlier tactic against the hydra was a good one, so we attempt it again.
All proceeds according to design up until the throw. The rogue rolls two crit fails in a row, and succeeds in impaling his foot with the demon dagger, then rolls a one on his save against the demon. He screams as his soul is torn from his body, which proceeds to crumble to dust mid-scream. The dragon takes the dagger and proceeds to massacre the entire party.