- An Iai skill used with a serene mind. Deals extreme damage when Undine is summoned. In addition, it can forcibly end the Undine summon.
- “Listen, first you need to put your sword back into your sheathe. Then entrusting your heart to the flow, you draw it and strike forward in the blink of an eye. In other words, it's the ultimate Iai attack that can only be used with a serene state of mind. An attack unmatched in speed that can only be performed with a serene state of mind... Its name is Serene Demon Sword.” - Alice
- “Now then... This skill is a little bit special, so listen to me. Of course you can use this skill like normal. It only takes 2 SP, and it's quite strong. But be careful when using it with the power of water summoned. It will be powered up, but will also drain your concentration. When you use the skill, it will forcibly end your serene state of mind. Don't you remember the effect when you use the power of water? The effect goes away when you're out of SP. In other words, when your SP goes to 0, you go back to normal. In other words... It would be almost suicidal to have the power of water cut at a bad time. Like that, you can protect yourself from a bad situation.” - Alice
- “The Monster Samurai Elreena cut an entire group of attackers in half in an instant with that move. She was said to have calmly sheathed her sword as their blood bloomed like bright flowers from their standing lower bodies.” - Alice
Serene Demon Sword (魔刀・明鏡止水 Matou Meikyoushisui) replaces Demon Decapitation when Luka learns it. It is a water and an iai type of skill that pierces through an enemy’s defense with ease. It is also known to strike far beyond a normal attack range. After the third battle with Granberia and Luka finished recuperating, Alice teaches him this skill as a “reward” for mastering the Serene Mind. Notably, Luka seems to master the technique far more quickly than others before, able to perform the attack the moment Alice tells him about it, even scaring her straight off the bat and leaving a huge gash in the wall of the room he was practicing in.
Drawing the sword from the sheath and attacking with a fluid slash, it’s a 2 SP skill which does higher damage than a normal attack. Combined with Undine, this attack becomes significantly more powerful, and cancels Luka out of his Serene Mind state. At Undine Level 3, Luka is not cancelled out of his Serene Mind state, and simply does more damage with her power used. Overall, it is his most powerful technique, even above Vaporizing Rebellion Sword in terms of SP cost.
Despite replacing Demon Decapitation, Luka is forced to resort to that skill after defeating Archangel Ariel, as Promestein disables his elements, but is soon replaced by Daystar while fighting Archangel Ranael.