Location
The flurry rune can be found in the Teal-Green Grass (starting) area by returning to the bridge just before the second boss, Jin, after the prologue. It is guarded by the boss Berserker.
Properties
Equipping the flurry rune causes Shin to attack with his paws instead of his sword. This deals slightly less damage than a sword attack, but allows him to attack many times per second. Gaining the Triple Slash upgrade adds an extra hit with each button press while using the rune, increasing its damage potential. With triple slash, a 3-hit attack deals .9/.8/.5 versus the standard sword damage values 1/.75/1.
Upslash and downslash are not affected by the flurry rune.