Lord Harkon

Lord Harkon is the main antagonist of the first add-on of The Elder Scrolls V: Skyrim, Dawnguard. He is a nord vampire and the leader of the Volkihar Clan, the father of Serana and the former husband of Valerica.

Background
According to Harkon himself, he was once a mighty king with a vast domain, but his greatest fear was his own mortality, so he pledged himself to Molag Bal, and sacrificed a thousand innocents in his name, and Molag Bal rewarded him by turning him and his family into pure-blood vampires.

Over years of ruling Volkihar, he discovered a prophecy, "the Tyranny of the Sun" that if enacted, would block out the sun and allow vampires to roam freely during the day. He became so obsessed over fulfilling this prophecy, that he pushed aside everything in his life, including his family, to enact the prophecy. In order to enact the prophecy, he required an ancient weapon called Auriel's Bow, and the blood of a Daughter of Coldharbour, he would then put the blood on an Elven arrow and use the bow to shoot it at the sun. Since his daughter, Serana was a Daughter of Coldharbour, he wanted to use her blood.

In order to protect Serana from Harkon, his wife Valerica locked her in Dimhollow Crypt along with the Elder Scroll of Sun, and escaped to the Soul Cairn with the Elder Scroll of Blood. Enraged by Valerica's betrayal, Harkon commanded his vampires to look for them.

EVENTS OF THE ELDER SCROLLS V: SKYRIM
After the Dragonborn finds Serana and returns her to Castle Volkihar, Harkon offers to make them a vampire lord and join his vampire clan. if the Dragonborn refuses his offer, Harkon will banish them from Castle Volkihar.

If the Dragonborn sides with the vampires, Harkon sends them and Serana to find someone who can read the Elder Scroll, they soon find a Moth Priest named Dexion Evicus, who is capable of reading Elder Scrolls, and bring him back to the castle. They then discover that two additional Elder Scrolls are needed to fulfill the prophecy, the Elder Scrolls of Blood and Dragon. Harkon then sends the Dragonborn and Serana to find them.

The Dragonborn and Serana eventually realize that Harkon's plan to block out the sun will doom both mortals and vampires, as it would bring too much attention to vampires, and they would be wiped out by mortals, while mortals would suffer without sunlight. Once the Elder Scrolls have been decyphered and Auriel's Bow has been obtained, the Dragonborn and Serana go back to Castle Volkihar to face Harkon, where they battle and slay him, putting an end to his tyranny for good.

TRIVIA

 * While his family are all vampire lords, Harkon is the only member to visibly assume his vampire lord form.
 * No matter which side the Dragonborn chooses during Bloodline, Harkon will always be the main antagonist in the end.