The Elder Blood is a book obtainable in The Witcher 3: Wild Hunt.
What exactly is the Elder Blood?
According to some, it is a powerful elven curse passed from generation to generation. Those in whose veins the infamous Hen Ichaer flows are said to carry death and destruction within them, to sow hatred and disdain in the hearts of men. It was from this contaminated blood that Ithlinne prophesied an avenger would be born, a destroyer of nations and worlds.
Others claim the Elder Blood is an extremely rare inherited talent, granting control over time and space to a degree unattainable even to elven Sages. Sadly, few bearers of this gift have been able to control it in full. This merely partial mastery inevitably leads to sudden outbursts of the Power that take the form of unpredictable, uncontrollable and thus extremely dangerous explosions. Behind the dark legends about the Curse of the Elder Blood lies the truth of these tragic cases.
For obvious reason the bearers of the Elder Blood were always subjects of great interest to the world's mighty and to mages alike. The former counted on the truth of Ithlinne's prophecy, on an avenger being born who could destroy worlds, and who they could thus use for their purposes. As for the latter, they hoped to harness the magic of Hen Ichaer to broaden their own knowledge and powers. Ultimately, however, all these plans were for naught - the Elder Blood line broke off with the disappearance of the heiress to the Nilfgaardian crown, Cirilla Fiona Ellen Riannon...