The Ash of War imparts Quality affinity to an armament and introduces the skill "Storm Blade."

Engage the lost skill of Stormveil by enveloping the armament with cutting storm winds that can be discharged forward in rapid succession.

Usable on swords, excluding colossal weapons and twinblades.

To secure these Ashes of War, explore the broken house of the warriors of Nimgfu.

Alternatively, you can obtain this Ash from  Bernal by exchanging 1800 runes for it.