Sentience. A trait possessed only by intelligent creatures. A trait thought impossible to find in any automaton, even with the most cutting-edge technology and the most advanced magics. A trait that, when mixed with immortality and solitude, can make someone quite the weapon. A weapon which a pony would do anything to get.

Princess Cadence feels a surge of powerful magic energy coming from the lower crystal wilds and, feeling concerned for the well being of her subjects, asks for help from the Princesses, who in turn send out the elements of harmony to investigate. What they find is what nobody could have ever expected.

This story takes place right after king Sombra's defeat, so Twilight is not a princess.