An anvil, atop a stone, has a sword stuck into it.
There is a plaque beside it that reads "Whosoever pulleth out this sword from this stone and anvil is the trueborn king of the realms".
Removing the sword requires a Strength check 20. If a player successfully pulls the sword from the stone, the sword comes to life and fights all players until it is destroyed (STR 13, DEX 18, CON 15, INT 1, WIS 2, CHA 1, AC 17, HP 5d6, speed 50ft flying).
The plaque was, obviously, incorrect.