The Blood of Hailes was the name for a holy relic, ostensibly a phial of Christ's blood, on display at Hailes Abbey in Gloucestershire, England (The Dark Secret of Weatherend).
Edmund, 2nd Earl of Cornwall, donated the phial of blood after purchasing it in Germany in 1270. The blood was eventually declared a fake, reported to either nothing more than a mixture of saffron and honey or animal blood, and disappeared following Henry VIII's Dissolution Act.
The Dark Secret of Weatherend
A "small, glass tube" capped at both ends with silver and containing a dark reddish substance was discovered hidden in the casket of J. K. Borkman. When handled by his son, Anders, the holy relic reacts with the "utterly, totally demonic" Anders and the two destroyed each other (Dark, 166). Emerson assumes the elder Borkman kept the relic as something that could counter his plans to destroy the world. Later Emerson Eells reads in the elder Borkman's papers at the University of Minnesota that he bought the relic from a "crooked antique dealer" near Hailes (Dark, 178).