She promised to go back once she lived out her life in Aboveground. She only promised this because the fates predicted she would die in child birth, swapping her life for her child's. Once her eyes closed in the Aboveground they would open again in a new shell. A new fae body, fit for ruling and protecting her kingdom in the Underground as it's long awaited Queen. Everything the fates predicted came true, she thought as the crushing sadness weighted her down into the arms of her King.