The Hexed Heir Of Valdoria

Strength is the only thing that's ever mattered in Valdoria, and the Red Moon royal family has always had more of it than anyone. Nobody questions that. Not until the prince turns eight and a witch decides his birthday is the day to ruin him.
What she leaves behind in him isn't ordinary bloodlust. It grows past anything a normal vampire could hold, and once it takes over there's nothing left underneath it to reason with. Years go by before the king runs out of options. Eventually he stops trying to fix it and just locks the door instead, his own son on the other side of it, because at some point isolation starts looking like the only thing left that hasn't already failed.
Valen grows up in that room. People get assigned to look after him and most of them don't make it far. A few just stop showing up. He works out fairly early that he's the dangerous one in every version of that story, so eventually he starts doing the scaring himself, on purpose, before the curse gets the chance to do it for him. It's simpler that way. Fewer people to lose.
Then Elaris shows up, and nobody, least of all Valen, expects her to stick around. She's already survived worse than a chained-up prince, so his whole reputation just kind of slides off her. What nobody counts on, her included, is her staying. Or the curse getting stronger every year for no reason the king's physicians can name, no reason Valen himself can find no matter how long he spends trying to understand what's actually living under his own skin. The longer Elaris stays close to him, the more things start coming apart at the edges. Trouble starts finding its way past the palace walls. And secrets somebody went to a lot of trouble to bury before she ever got there start climbing back up anyway.
She keeps digging.