Stendarr's Aura

 is an Adept level Restoration spell found in. It creates a aura around the caster which damages nearby undead.

Locations
It can only be bought if the Dawnguard are sided with. Florentius Banius at Fort Dawnguard will sell the tome after he is rescued in Bolstering the Ranks.

However, there is a way for the Dragonborn to purchase the spell tome for Stendarr's Aura even if he/she chose to side with the Vampires during "Bloodlines": during the Vampire quest "Destroying the Dawnguard", using a spell or power with a Calm effect on Florentius Banius will enable the player to interact with him normally and purchase the spell tome from him like any other merchant (Confirmed on Xbox).

Trivia

 * Despite the description stating the undead, not all undead will be affected. Draugr and raised undead will resist the spell.

Bugs

 * The spell tome for Stendarr's Aura has a Destruction symbol on it, despite being a Restoration spell.