Humiliate her and call for an armistice.

The guards nod at your command, strip Shalia of all her clothes, beat her a little bit to keep her weak, and bring her to the feet of the invading army. Both the men and women of the elves gasp at the sight of their naked hero.

“I understand that this woman was supposed to be some kind of ‘chosen one’ among your people.” You say. “Look at her pathetic state now.” You kick her in the side, knocking her over from standing on her elbows. “I am willing to forgive the lot of you for following such a foolish endeavor, and you are all free to go home on the condition that I establish an armistice with your leaders.”

An elf man steps forward. He bears a strong resemblance to Shalia, with blonde hair and green eyes. “I am Ehlark Valyarus, leader of this tribe and Shalia’s father.” So you were right. “We humbly accept your offer for peace, and I will take Shalia with me-”

“You misunderstand.” You interrupt. “This isn’t a peace offering, this is an armistice, a temporary reprieve from fighting. Additionally, your daughter is staying with me as a prisoner of war.”

Ehlark is taken aback, and the other elves murmur among themselves. Clearly, none of them has been to war, despite their alleged longevity. Ehlark finally nods, “Very well. She will stay with you-” he narrows his eyes at you. “I sense a contract spell has been cast on you…” He looks toward Shalia, “… and my daughter.”

“Yes,” you explain. “She had cast the spell on the contingency that as long as I do not harm this one here.” You point to Sorisana. “I can do whatever I like with her.”

Ehlark’s eyes widen. “What, exactly, were the terms of the contract you cast?” He says sternly.

Shalia coughs and looks at her father weakly. “As long as he does not harm Sorisana, I will do whatever he says.”

Ehlark says nothing, waiting for more. Nothing more came. He sighs and rubs his hand down his face. “And you didn’t create an escape clause or an end condition?” he asks, frustrated.

Shalia smiles weakly.

Ehlark straightens himself. “Very well, you may keep Shalia and do with her as you please, but her retainer, Sorisana,” he points to the raven-haired elf, “must stay close by and attend to her often. In the meantime, the rest of us will head home to discuss a peace treaty with your nation.”

You nod. “I accept your terms.”

Everyone goes their separate ways. You have Shalia locked in a cell in the dungeons and give Sorisana a guest bedroom. You order your castle staff to treat Sorisana with respect, and anyone who harms her will be met with swift and harsh discipline.