Blessed Wizard Oil

Blessed Wizard Oil is given by the Argent Dawn in exchange for during the Scourge Invasion. When the oil is applied to a weapon, the wielder will have increased spell damage against the undead.

Source
This item is a quest reward for several Scourge Invasion quests:

It is also available for purchase for 8 Necrotic Runes from any Argent Quartermaster after completing Under the Shadow.