She was a Nedic tribeswoman who was raped by the Daedric Prince of domination and enslavement, Molag Bal. It is recorded that after the ordeal, Molag Bal shed a droplet of blood on her brow and left her to die.
Lamae was found by nomads and cared for, however during the night she passed into death. As her funeral pyre was still burning she emerged as the first pure-blood vampire and brutally killed the nomadic tribe.
As she was the first individual granted Vampirism directly from Molag Bal himself, she became known as a Daughter of Coldharbour. This developed into a tradition among Molag Bal worshipping cults which dictated that females be offered to him on his summoning day. Few survive the ordeal and those who do emerge as pure-blooded vampires. Being selected as an offering was considered to be an honor that is not rejected lightly. 
Lamae spread her vampirism across the land and became the progenitor to the Cyrodilic bloodline of vampires known as Cyrodiil Vampyrum. She was not responsible for all vampiric bloodlines, however, as Harkon, a Nord warlord, is considered the father of the Volkihar bloodline found in Skyrim.