The input for this move is b+3,1,4,3. There is also another variation of this move called Demon's Wrath with an input of b+3,1,4,1.
When to use
As a character that lacks the necessary tools to put pressure on an opponent, Kazuya only makes up his weakness with a few fast poking combos. The Reign of Terror and Demon's Wrath can be mixed up during close quarters combat, confusing the opponent as to whether the last attack will be a mid or a low.
The first two hits of both move lists can be used for mixing up combos, which in turn rewards the player with a lot of pain dished out against an opponent. A guaranteed combo mix-up for this could be WS 2, b+3,1, d/f 1, 2, f,N,d,DF+4,1.