Prince Constel, brother to Celestia, and Luna, the one who truly holds the stars in the sky, turned vampiric from his escape from the prison in the moon with his father has fallen in love with Sapphire Shine, the last pony he must devour's soul, from a distance, and must choose between his love for the young colt, and his desire to gain enough power to release his father from their once-shared prison, and rule the world alongside him.
The bloodlines of the main 6 have all ended, save Twilight Sparkle's, and if he kills Sapphire, the last of the bloodline will fall, and he will have the raw power of the Elements of Harmony, and be able to corrupt them into Elements of Hatred. Giving him the power to release King Dark.
The reason for the gore warning is that in every numbered chapter, not the prologue, there will be an instance of what I can only describe as cannibalism, as Constel kills, devours the soul, and then feeds off of the flesh of a pony, to survive until another night.