Aria Shadewood was an unwilling Vampire, attacked by a Vampire Lord in the events following the Oblivion Crisis. Believing herself cursed, after realizing she was turned into a Vampire, she fled into the wilderness, only kept alive when, out of starvation, she'd attack the occasional bandit camp (and even then, she'd be overwhelmed with guilt from it). Also, she was protected from Vampire hunters by her former compatriots during the Oblivion Crisis, including Mage's Guild Archmage Nola Maenn.
Eventually, she was taken in by the Alliance of the Dovah, their leader Cynthia Maenn (Descendant of Aria's old friend Nola Maenn) doing what she could to alleviate Aria's pain. At that point, Aria Shadewood decided to do what she could to use her 'curse' to aid the Alliance's cause. However, this would cause no end of trouble between the Alliance of the Dovah and the Dawnguard, the latter not being particularly happy with a vampire being in the Alliance's ranks.
She has an almost obsessive fascination with ancient civilizations, and is the Alliance's most bountiful source of information on the Ayleid Civilization and Magic, as well as second only to Krazhir when it comes to information on Dwemer Civilization and Technology.
Somehow, Aria Shadewood took on the ability to transform into a Vampire Lord from her attacker, and, over time, her Vampire Lord form had been enhanced with Dwemer Augmentations, leading to form a rather unique transformation.