Rapid Slash, or Shissou Iai (疾走居合, lit. "Dashing Re-sheath") in Japanese, is one of Vergil's special moves. Vergil charges forward and slashes rapidly via creating a ton of rift-based slashes with iaidou. This move actually hits you from behind, due to the fact that the actual hitbox comes out later when Vergil reaches the end of his dash.
It is due to the startup of this move and when the hitbox appears, it can be easy for Vergil to be intercepted by certain moves such as throws if he uses this recklessly.