When a medieval princess releases an evil spirit that intends to murder her husband, she must find a way to destroy the spirit and save her husband.

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

The story starts when the spirit threatens to kill her husband - as inciting incidents go a threat is fine but it can be better. What if the spirit were to kill the husband and try to kill her child? Now the inciting incident is much more significant and the threat gains some weight.

Best to reword the logline so it starts from the threat to kill and then describes the action she takes to eliminate said threat - destroy the spirit, banish it to hell, or catch it in a Ghost Trap (who you gonna call...).

Ultimately, whatever it is she does needs to be described in the logline in more detail than it is now.