Claude beat the shit out of kusoprince
Aileen, realizing where this is going in the game, tried to calm him down.
He still got too emotional and transformed into dragon.
He has the Beast's 'don't look at me' self-hatred moment.
Thot came in 'oh pitiful demon lord, I'mma finish you off nao'
Aileen tanked the stab, something about the heroine losing her qualification for using the holy sword on a human, and that Aileen actually is related to the past holy maiden, so she basically 'stole' the sword from the Thot.
Aileen then managed to make Claude turned back to his human form (naked, causing her to blush and look away)
Then the two got married.
Aileen's dad basically convinced Claude like this.
Claude: I don't want to be king.
Dad: Well, y'see, she will marry whoever become king.
Claude: I will become king then.
Although it's not the end of the story apparently :v from the comments it seem it's the end of vol 1 of the LN, and from vol 2 onward is the fluff married life with those two.