When Princess Celestia asks Twilight to make an official, royal visit to Romaneia, Twilight dreads the inevitable succession of dry, boring diplomatic events. The charming Lord Bela soon wins her over, but Twilight is about to discover that all is not as it seems in Bela's apparently peaceful castle. (A story inspired by Carmilla, Dracula, and other classic vampire tales.)
Written for the BadassTwilight = Total Domination Twilight's Nightmare Halloween contest.
(Spoilers in comments.)