The elven king has been killed and the queen captured, her fate is to be your plaything.
After centuries of war between the human and elven kingdoms, the humans undertake a successful operation that leads to the elven king's death, with the queen as part of the spoils returned to the humans.
In order to perhaps teach a haughty elf humility, or simply just to degrade and debase her for the crimes committed against the humans, the human king assigns a unique punishment to the now ex-queen: she is to be the young prince's plaything to do with what he likes.