Odeseeker (the strongest elf) and the Elf Queen were supposed to officially bang to produce the next Elf ruler, but one of them was sterile. Odeseeker was kicked out of the palace and the Queen was imprisoned in it as a failure (though based on the author's constant NTR writing it was most likely Odeseeker's defective balls that was at fault).
Odeseeker swore to burn down the kingdom as payback, made a homunculus clone of the Queen as their "daughter", and turned her into a super weapon to do it.
MC's jailbait became best friends with the clone, so the MC beat Odeseeker and took over the government, since being contracted to multiple great spirits at once made him stronger than the royal bloodline.
The MC freed the now former Queen and both her and Odeseeker swore allegiance to him as thanks for reuniting them and sparing their "daughter" (also for the MC not fucking Odeseeker's wife in front of him lol).