Heavenly Strike is a Mythic Technique, a single blinding horizontal slash you learn by finishing the Mythic Tale The Heavenly Strike. It cuts through guards and is deadly against Staggered enemies.
Stagger the target first, then strike for huge damage. In duels it works as a fast, low-damage poke, the opposite of Dance of Wrath.
With the Way of the Flame active, enemies who survive the slash also burn. A New Game+ Charm, Heavenly Rebuke, can make lightning hit a nearby enemy.