Abducted Princess Running Rampant: Wielding Forbidden Magic in the Demon King's Castle

"As a holy princess born with divine powers, Mil spends her days blessing adventurers from all across the kingdom. Inspired by their tales of dragons, demons, and treasure-filled dungeons, she has long dreamed of someday setting out on an adventure of her own. But she’s destined for an uneventful life confined to her father’s castle. That is, until she’s kidnapped by the Demon King and forced to use her powers to aid his forces instead! Unfortunately the novelty soon wears off, and life imprisoned in the Demon King’s Castle turns into another dull routine. It seems Mil’s only hope to escape the monotony is to somehow grow strong enough to break free and evade her captor. In a castle filled with demonic combat instructors, cursed weapons, and tomes of evil spells, Mil’s development is going to be far from holy, but that’s not about to stop her. She’s determined to have the adventure she has always longed for, even if it means mastering forbidden magic!

[From J-Novel Club]