Blessed Wizard Oil is given by the Argent Dawn in exchange for 8 16px during the Scourge Invasion. When the oil is applied to a weapon, the wielder will have increased spell damage against the undead.
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.
While this item can be used any time, you can only earn more Blessed Wizard Oil during the Scourge Invasion.