Samloyal23
Adventurer
How about a combination grey-goo/midas-touch scenario?
A botched attempt to create the Philosopher's Stone leads to an uncontrolled chain reaction: Anything the stone comes into contact with turns to gold, and anything that comes into contact with something the stone is touching also turns to gold. Anything touching the secomd object turns to gold as well, and so on, and so on, as long as an unbroken chain leading back to the stone exists. Left unchecked, this will rapidly convert the entire planet into gold.
This would be a great way to become rich and destroy your enemies at the same time. An enemy from another world could launch at your planet then watch the mayhem as people at first fight over the gold, then realize their error as their world is slowly converted into gleaming, yellow metal one square mile at a time.