A secret fallen angel skill to cut through space and time. It has very high damage for low SP use. It seems to be compatible with the power of wind...
"The blade of judgement comes for you!" - Luka
"Cutting through time, space and reason!" - Luka
Flash Kill (閃殺) is another skill Luka uses when under the influence of holy power. It is a single slash that apparently has the power to distort space and time, as Luka was able to cut and destroy Kraken's Aqua Pentagram, an ultimate defense ability that also utilizes the power of space and time.
After Luka finally awakens his angelic powers in Chapter 3 while fighting Archangel Ranael, it replaces Lightning Sword Flash and costs 2 SP to use. Unlike its predecessors, it does not have a damage boost when used on the first turn, but instead behaves much alike Demon Decapitation/Serene Demon Sword.
Once the seal weakening the four spirits was removed, Sylph powers up this skill to Flash Kill: Destructive Wind (閃殺:絶風), making it much stronger.
This skill was also used by Marcellus and Nero from Monster Girl Quest Paradox hinting that they learnt the same skill on different timeline.