Tyr's Hand Holy Water

The Tyr's Hand Holy Water was required by the Blood Knights because the Scarlet Crusade's misguided zeal was so strong that it had even tainted the blessing spoken over this water, making it perfect to prepare the mixture that was to extinguish the eternal flame of the Alonsus Chapel in Stratholme.

Source
Contained in Basin of Holy Water in the Library Wing of Tyr's Hand Abbey.

As a quest objective

 * True Masters of the Light