Kū Soku ZIX (クウソクZIX - Kū Soku ZIX) is one of Shiki's Secret Ninja Arts that appear throughout the Senran Kagura series.
Shiki takes off her hat and tosses it up into the air, where it expands. Her hat turns upwards and releases a ripple-like wave of darkness that carries many of her bats towards her opponent(s).
The name "Kū Soku ZIX" is a pun based on the Buddhist saying "Kū Soku Zeshiki" which means that all things, matter, mind, and events, are produced by Kū, which literally means "emptiness" or "void", but has a connotation of equality. The phrase goes hand in hand with the phrase that Shiki Soku ZEX is based on to basically mean that all phenomena are produced from an equal kind of energy or force.