Lord Harkon is first introduced in the series when Fün takes Serana home to Castle Volkihar. After being let into the castle by a watchman, Fün arrives at the main hall and sees Serana's daddy Lord Harkon feasting on humans.
Lord Harkon attempts to introduce himself but Fün already knows who he is. After all, it was written on the floor right by him all the time. Wanting something for having brought Serana home, Fün questions about a possible reward. Lord Harkon then replies that besides the Elder Scroll, he can also give her something quite valuable, his blood. Immediately he turns into his vampiresque form, making Fün think he has a massive demon dick underneath his garments, with a barbed end and a smaller dick inside that shoots out. As soon as he transforms, he bites Fün and turns our Pimp into a vampire as well.
When Fün wakes up, she's all sparkly and Lord Harkon says that, with the proper guidance, she can become a striking instrument, to which she chose to be a trombone with a blade at the end. She, herself, turns into her vampire form and plays around for a bit, becoming a cloud of bats for a minute and then redirecting her attention to Harkon, who says he has a task for her and sends her to Garan Marethi. Fün does not like his nose nor facial hair and is intrigued by the fact that he keeps on reading when talking to her. He might be reading a script... Garan sends her to find a chalice Harkon seems to want or something. (Huge Vampire Donk)
After finding the the chalice that Harkon wants, Fün returns to Castle Volkihar and speaks to Lord Harkon, asking if his daughter is available for boning to which he replies affirmatively. Harkon tells Fün their biggest enemy is the sun and pretty much asks her to kill it. After not hearing his proclamation, Fün goes to find a Moth Priest. (Stupid Moth Priest)
After shooting the Sun with Auriel's Bow, Fün and Serana kills Lord Harkon. Fün overpowers him and no one, including Harkon, is ever a challenge. (Grudge Match of the Century)
In Arrow to the Face, Vingalmo speaks some vampiric nonsense to her and she feels the need to ask him why is she able to bestow the gift of Harkon's blood and if there's any way she can bestow his urine. She finds out that for such, Harkon had to had peed on her face and then follows up questioning about the possibility of herself peeing on someone else's face and bestow them the power of vampiric urination, having that happened. Fün discusses with her brain tumor, Emre, why don't vampires bite people in the ass and turn them, but Emre says it would be stinky, which leads Fün to understand why they do it on the neck. (Arrow to the Face)