Skyrim: Random Encounter - The Vampire Assassin
This random encounter can occur after completing the Dawnguard quest "A New Order". While traveling one of Skyrim's many roads, you may come across a randomized female vampire, who appears abruptly and immediately attempts to kill you. After defeating her, you'll find a note on her person, revealing that she was sent by a vampiric patriarch who resides in a randomly chosen dungeon. Reading the writ prompts the game to mark the vampires' location on your map, leading you right to their doorstep for a little payback.
My individual encounter with the vampire assassin was quite humorous, as for some reason, she decided to pick a fight with two nearby mammoths and their giant owner. Not wanting her to kill the mammoths, I used a Fortify Alteration potion to increase the duration of my Slow Time shout, allowing me to defeat her easily.