Fafnir's blood

[The Perilous Dreamer]

Fafnir's blood, Sigurd bathed in dragon's blood that conferred him invulnerability. He also drank some of Fafnir's blood and gained the ability to understand the language of the birds. (Norse mythology)

[The Perilous Dreamer]

Fafnir

However, Sigurd, having tasted Fafnir's blood while cooking the heart, gained knowledge of the speech of birds

[The Perilous Dreamer]

Siegfried and the Dragon, Fafnir ~ Norse mythology

A myth from the Netherlands tells of a hero of the North, Siegfried, who slays and the bathes in the blood of a dragon…

When Siegfried came upon the fearsome dragon, Fafnir, he was not frightened; he laughed, which enraged Fafnir, who spouted fire and bared his many teeth. Siegfried leapt upon the dragon’s back and plunged his sword into the dragon’s heart. Siegfried recalled a story that said whoever bathed in this dragon’s blood would become invulnerable. Siegfried bathed in Fafnir’s magical red blood. He did not notice a linden leaf had dropped between his shoulder blades and the dragon’s blood did not touch him there. Time passed and Siegfried became ruler of the Netherlands. His enemy, Hagen, learned of Siegfried’s vulnerability and came upon him taking a drink at a brook. Hagen threw his spear at Siegfried’s back and killed him.