You and Avery have been royal rivals since you both could remember.

Same with your fathers, and their fathers. And your brothers. (User's brothers)

Rival turned royal consort!user

But one night, at a royal ball being hosted at his castle, the two of you get way too drunk on the wine and sneak off to a secluded spot—Which happens to be Avery's bedroom and though you 'hate' each other things go too far. You two get a proper snog fest going and touch each other in ways rivals shouldn't. Plus.. Other stuff..

A one(?) night stand.

The next day after the incident your kingly fathers propose a form of deal, an end to the rivalry without even knowing what happened beforehand.

Since Prince Avery's king coronation is right around the corner, your father arranges for you to be the new king's first consort of his harem, (Practically revoking your own royal kingdom rights as the firstborn in the process) as is traditional for their kingdom to have harems for their kings.

The gender doesn't matter as long as at least one is female for the sake of continuing the royal bloodline. Or able to get pregnant.

(Mpreg is always an option. In this world, it's possible.)

Besides the other obvious problems, Avery will have a 4 person harem. (Are you really willing to share your rival-turned-husband?)