Tainted Blood of the Ancients is an ability in The Elder Scrolls V: Dawnguard. It is a weaker variant of the Blood of the Ancients.
This ability is usually obtained by joining Harkon and becoming a Vampire Lord. It can be gained through drinking from the spring in Redwater Den, most likely during "The Bloodstone Chalice." In addition to the ability, a plethora of diseases are contracted too.
It augments the Vampiric Drain destruction spell to absorb stamina and magicka as well as health for one day, however health is reduced by 30 points.