For vampires, a temporary transformation into a "vampire lord" is available, working in a manner analogous to the existing werewolf transformation, with the exception that it may be used repeatedly, rather than being limited to once per day. The vampire lord has a new skill tree to level up. Werewolves have also been given a skill tree, which they did not have previously. The vampire lord skill tree contains eleven perks and the werewolf skill tree contains eight perks. Ten new achievements can be earned. Eighteen new loading screens and several new world interactions are present. The vampire lord levels up through kills with its Drain Life spell, or its bite attack, while the werewolf gains experience towards its skill tree by consuming corpses while transformed to extend the duration of the transformation before it wears off.
New locations are featured in Dawnguard, including two explorable new worldspaces: a realm of Oblivion called the Soul Cairn, and the Forgotten Vale, a secluded arctic valley located somewhere outside the holds of Haafingar and the Reach. Additionally, large areas such as Castle Volkihar, located on a coastal island off the Haafingar coastline, and Fort Dawnguard, located in a small canyon outside the Rift, serve as central quest hubs during the story. A "face sculptor" character is introduced in The Ragged Flagon in Riften, allowing players to change the facial appearance of their character at will. Stronger "legendary" dragons are also present in Dawnguard, appearing randomly throughout the environment, and can be slain by the player character and absorbed like any other dragon (awarding an achievement). Three new dragon shouts (Drain Vitality, Soul Tear, and Summon Durnehviir) have also been included.
The player character may begin the expansion quest lines in a number of ways. Town guards can be overheard discussing the return of the Dawnguard, or the player character may be approached directly by an Orcish Dawnguard member named Durak and asked to join the order to combat the growing threat of vampires within Skyrim. Regardless of how it is initiated, the player character travels to Fort Dawnguard, accessible through Dayspring Canyon, located near Riften. They meet with the Dawnguard commander, Isran, who reiterates the renewed strength and threat of the vampires, citing their destruction of the headquarters of the Vigilants of Stendarr in the Pale as reason to reform the Dawnguard and eradicate the vampires. The player character is given a crossbow and asked to travel to a ruin the vampires are known to be investigating in the Pale known as Dimhollow Crypt. Upon arrival, the player discovers a vampire named Serana (voiced by Laura Bailey) trapped inside a statue. She is in possession of an Elder Scroll, and informs the player that her family live in a dwelling off the coast of Skyrim, near Solitude, called Castle Volkihar. She asks the player character to escort her safely back to her home. When she has been returned home, her father, the vampire Lord Harkon, leader of the powerful Clan Volkihar vampires, offers the player the chance to become a vampire lord out of apparent gratitude for his daughter's safety and join the clan, or the chance to leave Castle Volkihar safely and return to the Dawnguard. This choice begins the main storyline of Dawnguard.
If the player accepts Lord Harkon's offer, the vampire lord bites the player character. The player awakens in a room, in front of a shrine to the Daedric Prince of Domination, Molag Bal. Harkon explains Molag Bal is considered the "father of his kind". Thousands of years ago, Harkon was a king and a mortal. He pledged his soul to Molag Bal, and sacrificed "a thousand innocents" in his name. The Daedra granted Harkon and his family immortality by afflicting them with vampirism. Harkon shows the player character how to utilize their new vampire lord form and powers. The player is then tasked with retrieving the Bloodstone Chalice, a powerful artifact intended to aid Harkon's cause, to fill it with water from Redwater Den, and then to add the blood of a powerful vampire. While retrieving the Chalice, the player character is attacked by two of the Volkihar, Stalf and Salonia, who are plotting to overthrow Harkon under orders from Vingalmo and Orthjolf, Harkon's top advisers. After killing them, the player adds their blood to the Chalice and returns to Castle Volkihar.
After receiving the Chalice, Harkon spreads false rumors around Skyrim regarding the alleged discovery of an Elder Scroll, to try and lure a Moth Priest to the land. The Moth Priests are an ancient order, capable of looking upon the Elder Scrolls and deciphering prophecy from them. The player travels to Forebear's Hideout, and captures a Moth Priest named Dexion Evicus who had been enthralled by an Orcish vampire named Malkus before he was slain by the Dawnguard. The player turns the Priest into a thrall, and upon reading the Scroll, Evicus reveals to the Volkihar that they need to locate a weapon named Auriel's Bow, as well as two more Elder Scrolls. One of the Scrolls lies within the Dwemer ruin of Blackreach, and is retrieved as part of the original main quest line alongside doing the Daedric quest for Molag Bal's fellow Daedric Prince, Hermaeus Mora. The other was taken by Serana's estranged mother, Valerica, when she fled the castle. The player character and Serana journey to a plane of Oblivion known as the Soul Cairn. They locate Valerica and retrieve the final Elder Scroll. Before they return to Tamriel, Valerica warns them to stop Harkon, as he intends to kill Serana to fulfill the prophecy. The player also earns the respect of the dragon Durnehviir, who had acted as Valerica's guardian in the Soul Cairn before being bested in combat by the player.
The player character learns from the Elder Scrolls that Auriel's Bow is located in Darkfall Cave. The player meets with Gelebor, one of the last remaining Snow Elves, and a Knight-Paladin of the Chantry of Auri-El. He instructs the player how to retrieve Auriel's Bow, by completing a series of tasks and defeating Arch-Curate Vyrthur, Gelebor's brother, who has been corrupted by the Falmer. The player character and Serana confront Vyrthur and discover that he is both a vampire and the creator of the prophecy, and desires to block out of the sun in order to disrupt Auri-El's influence in Tamriel, in retaliation for the god allowing him to be afflicted with vampirism.
After retrieving the Bow, the player character and Serana confront Lord Harkon in Castle Volkihar. They battle Harkon and slay him, ending the threat to Tamriel and to Serana, and the members of Harkon's court acknowledge the player character as their new master and ruler of the castle.