The Unbreakable Oath

The audience

The Unbreakable Oath is the magical pact sealed between Magnus, the Demon King, and Shun Sato in the throne hall of the Demon Castle. It is imposed at the audience Shun had been awaiting since his arrest, after days alone in the castle dungeons.

Akiyama Magnus escorts the prisoner to the hall's doors but does not enter; the doors close behind Shun and leave him to face the king alone. Before Shun can take a step down the red carpet, Magnus drags him the length of the hall with an invisible, crushing force, suspends him before the throne, and snaps his manacles with the pressure alone. Ranks of guards and nobles of the Demon Court line the carpet as witnesses — the narration calls them the venomous jury who would judge and witness the fallen lord's execution.

“For love”

Magnus's first demand is a reason. He accuses Shun of discarding his loyalty and potential, and of sacrificing his honour and dignity for his “pitiful peace,” and roars “WHY?!” with enough force to shatter the hall's stained-glass windows.

Shun's answer, given after he has been choked from across the room, is: “Why do men do things others deem inconceivable? For love… of course.” As he says it, he sees Yumi Raijin laughing in her blue tulle gown. Magnus cannot at first believe it — he reminds Shun that he threatened the peace of both realms and the lives of millions, and that he is throwing away the very peace he betrayed Magnus to build and fought for over twenty years. Then the king laughs, and Shun laughs with him.

The first condition

Magnus asks Shun whether he wishes to live, and offers to spare him on two conditions, raising two fingers for the whole hall to see. The court's nobles, who want Shun dead, are outraged but too afraid of the king to protest.

The first condition is stated as a command. Shun is to find the woman, bed her, and when his firstborn child comes of age, hand that child over to the court to serve the Demon King until their final breath. Magnus frames it as wanting Shun back in the court “in a slightly different capacity.” The narration reads the demand as asking Shun to give up his own flesh and blood as the realm's next perpetual plaything, forcing an innocent child to suffer for unknown decades as Shun had suffered.

Shun counters with terms of his own: “Should my firstborn child prove stronger than the strongest royal knight, and defeat them in open combat… they are to be set free, never to serve any Demon Lord!” Magnus accepts at once, delighted, and calls it “a bet for the ages.”

The oath's recorded wording is **firstborn child**. The article preserves that wording and does not narrow it.

The binding

Magnus draws Shun through the air with gravity magic and offers his hand. When Shun takes it, the king squeezes, and a shockwave of magic of what the narration calls an unknown, primeval nature ripples out from their grip, shaking the foundations of the castle. Both men's sleeves are shredded, and a thick, pitch-black, inky substance swims up their bare arms, searing itself into the flesh.

The ink settles into two different marks:

- on Shun's arm, an intricate tattoo of **black, spiralling roses choked by sharp thorns**; - on Magnus's arm, a mass of **black, ancient roots** that climb until they anchor near his shoulder.

Both feel the same agony. Shun howls; Magnus laughs through it. When Shun demands to know what has been done to him, Magnus names it: “This is the Unbreakable Oath. Should either of us dare to violate a single term of our pact, the very same agonizing sensation we just endured will return. It will crawl across our bodies, unchecked, until it leaves us dead.” He presents it as the answer to Shun's own request to raise the stakes, and the narration calls it his perfectly executed trap.

The second condition

The second condition is one word: “Survive.”

Magnus strikes Shun off the dais with a backhand and brings down his gravity magic on him with the weight of a thousand mountains. Shun's joints and bones break and his organs are crushed; blood sprays from his mouth, nose and eyes. When Magnus adds his second arm, the flagstones give way and Shun is driven into a crater in his own shape.

At the edge of death, a separate power already inside Shun intervenes. A mocking voice he recognises at once speaks inside his skull — “Let me give you a hand… Little Shun” — and an inky liquid pours from the back of his skull, staining the whites of his eyes a dark grey. Unable to shield his whole body, this curse concentrates its protection on his heart, skull, ribs, and the bones of his right arm and foot, abandoning the rest.

The gravity field detonates. When nothing moves in the crater, Magnus sits back disappointed — “Yet another failure” — and begins, in the narration's words, calculating where his experiment had gone wrong and how to ensure the next candidate survived. Then the rose-marked arm reaches out of the dust, and Shun hauls himself upright: “I… Still… LIVE!”

Terms met

Magnus laughs until he coughs up blood into his hands, and does not care. “A deal's a deal, little stray! Return to your post!” The oath is sealed and both conditions are satisfied at the moment the chapter ends.

Undisclosed details

Chapter 23 closes on the king's order. How Shun leaves the castle, what the oath does after that day, whether its first condition is ever kept, and who, if anyone, the firstborn child proves to be are all undisclosed at this boundary. Magnus never names the woman; the only indication of whom Shun means is the image of Yumi in his own mind. So is the nature of the separate curse that speaks inside Shun, and the experiment and earlier candidates Magnus's thoughts refer to.