I'm inclined to go with something like:
Based on an ancient folk tale, when a medieval princess accidentally releases an evil spirit, she must find a way to destroy it before it kills her husband.
(28 words)
>>> Based on an ancient folk tale
The fact that it's a tale that's endured for centuries suggests it has "commercial legs"; if it's endured all this time, it might be a timeless (and profitable) story worth telling to a contemporary audience.
>>accidentally
It was accidental, right?? Or if she intentionally released it, she didn't know what evil she was unleashing?Whatever, I suggest insert the appropriate adverb to clarify her lack of malevolent intent.
fwiw