Prince Marse had already been derided as a failure by his father’s court before he was given One Job, to take up the mantle of the Ring King. That he couldn’t even do that, and with Princess Saphira forsaking him, he now feels completely worthless. The Oracle taps right into that baggage, unlocks his darker desire for power and redemption, and presents him with an ancient sword that can deflect the Ring King’s power.
Satou is busy getting chaste kisses from his wives when more monsters arrive at the palace doorstep, and that’s when Marse suddenly charges and slashes at him. Alabaster holds the monsters at bay while Satou and Marse duel, and Satou isn’t under any illusions he can beat Marse in a swordfight. But he can also tell something is off. He’s in as bad a shape as he should be.
Throwing caution to the wind like the fool Saphir believes him to be, Satou leaves himself open to what should be a killing strike from Marse, but the cut doesn’t go deep. That confirms it: Marse is hesitating. Marse falls to his knees and asks Satou to cut his worthless self down, but of course Satou can’t and won’t do that. Without Marse, Satou wouldn’t have lasted a day in this new world. If Marse can’t see worth in himself, Satou will give him worth: that of a knight of the Ring King who will save the world.
Once Satou breaks the gem on the pommel of Marse’s sword, he’s freed of the Oracle’s curse. The Oracle then uses a monster to take Saphira hostage. She’ll trade the princess for the rings, but Marse won’t hear of it. Accepting his new worth and resolved to live life protecting those precious to him, he proposes to Saphira right then and there, creating a distraction for Satou to slashs the tentacle restraining her. Marse catches her out of the air, as gallant and romantic an outcome as they could have hoped for.
Again declaring her new husband a fool—but the good kind of fool—Saphir kisses Satou and transforms into a massive water dragon that noms the Oracle’s comparatively puny monster. Defeated, the Oracle vanishes into a dark portal, while Saphir returns to her human form—completely nude, of course—and Hime helpfully shields Satou’s eyes.
With the Oracle’s hold on the king lifted, it’s revealed he’s a lot more youthful and vigorous than before (again, like King Theoden). He gives Marse his blessing to marry Saphira. Watching the happy couple embrace makes Hime a little envious, and then she and Satou blush together when Marse says “your turn”, implying he and Saphira have already consummated their love.
Even Sluder leaves on good terms with Marse, welcoming his brother home when he so chooses. However, their dad would probably less enthusiastic about Marse’s return. The Oracle, who he’d been working with, tries her controlling shit with him and he cuts her down, declaring that nothing, not he Abyss King or the Ring King, will keep him from achieving global hegemony. Well, we’ll just see about that, pops! One wife to go for Satou!