now, nonnie, you propose an interesting conundrum here.
on one hand, demon king rui defeating knight tsukasa is a tale as old as the demon king outfit has been out, BAHAHAH. 🤣🤣 but as they say, old is gold!
knightkasa is determined to defeat the demon king. it's his number one goal, and as a member of the white roses, he's made it his duty to slay the daemon that torments his land. now, there isn't very much information about how the underworld works - humans can only speculate so much - so when he eventually gets there and finds, well, more than one "demon king" ... needless to say he's a little confused of which one is the one he's trying to defeat. before he can even decide one, he's kidnapped, drugged, tied up, and brought to the black palace of one, rui kamishiro.
rui takes great interest in the knight. rarely do humans have the courage to venture into a world they could easily die in. he tries to speak to tsukasa to figure out what he's doing here, but tsukasa's a little bit too eager to fight. "oh, my. well... you should be aware, tsukasa-kun, that i'm not exactly a violent person. i have different means to fighting."
the knight is pinned down and fucked to oblivion by a cock that is anything but human. a shame fills his heart, but he loves it at the same time. how can demon dick be this good? well, of course it can be this could... they're demons... but never did he realize the temptation could be this strong.

conversely, imagine the same plot but with the rolls reversed. in an attempt to fit in with society, which has banished him for being an outcast, magician rui decides he'll try and bring back the body of one of the demon kings that has been plaguing the land. he knows a bit more about the underworld than the average human, and has decided his sights on a certain demon king named tsukasa.
tsukasa is not a malicious demon king by any means, in fact, he's quite "good." but that doesn't mean he doesn't still cause torment to the ones in the kingdom, whether it be voluntarily or involuntarily - a means to an end, if you will. rui makes his way to tsukasa's palace and tsukasa is startled to see the presence of a human, an actual human, in his underworld kingdom. "eh? it's impossible..!! how can a human be here?!"
he's shocked beyond belief, but when rui begins to conjure up some spells in his hand, he knows that he has to fight back. the two engage in a deep battle until finally, rui lands on top of him and holds him down by the neck. he could kill him, he could easily kill him right then but something inside of him just... snaps. he can feel a wetness down by the demon king's crotch.
suddenly, bringing the dead body of a demon king doesn't matter too much anymore. who knew demon pussy could feel this good? plus, the underworld seemed to suit him much better anyways.

i like both of the plots, as you can see. both sound interesting. and in both ways, tsukasa-kun is getting defeated, so i like it~ 🥰🥰