Arcane Catalyst

The Arcane Catalyst was required by the Blood Knights alongside Tyr's Hand Holy Water and other reagents to prepare the mixture that was to extinguish the eternal flame of the Alonsus Chapel in Stratholme.

Source
The Arcane Catalyst can be purchased from Zalle in Silvermoon City's Bazaar for.

As a quest objective

 * True Masters of the Light (2)