On an otherwise ordinary day, Princess Twilight Sparkle wakes up to find the town of Ponyville in absolute chaos. The entirety of the town, and more, has packed themselves into town square, all for a mysterious magical artifact that had seemingly appeared overnight.

A sword, elegantly gleaming in the sunlight, encased in a stone block with a simple inscription.

"Whoso Pulleth Out the Sword of the Stone and Anvil, is Rightwise Ruler Born of All Equestria"

Arthurian Legend in Equestria. There will be implied sexual activity, as is the nature of the mythology, but nothing explicit.