Hi CTH, it's a good start and could work as a marketing logline but if you are still writing the script then there are ways to improve it so you have a clearer vision for what the movie is about. This logline could be kept internal and doesn't have to be shared if you believe you have a really unique idea/character/villain/concept within the fantasy/mystery genre that no one has used before.
From your logline we can see that you have figured out the protagonist, location, genre, and inciting incident.
What you need to figure out to make your story better is the driving force, ie what is at stake, and the opposing force, ie what is stopping the protagonist from reaching their goal?
Let's take a look at some loglines from famous movies in the fantasy genre:
A reluctant Hobbit, Bilbo Baggins, sets out to the Lonely Mountain with a spirited group of dwarves to reclaim their mountain home, and the gold within it from the dragon Smaug.
A dark force threatens Alpha, a vast metropolis and home to species from a thousand planets. Special operatives Valerian and Laureline must race to identify the marauding menace and safeguard not just Alpha, but the future of the universe.
Four teenagers are sucked into a magical video game, and the only way they can escape is to work together to finish the game.
I have highlighted the important elements from each logline.
Hobbit: A reluctant hobbit, must reclaim a kingdom and its treasures from a powerful dragon otherwise he and his new friends will die.
Valerian: Two special operatives must protect their home planet from a mysterious dark force otherwise it will grow more powerful and destroy the universe.
Jumanji: Four teenagers must learn to work together and escape a video game otherwise they will be stuck inside it forever.
What will hook the audience is the strength of the concept and what is at stake for the characters, so if you can give them a reason to care for the story then you have a greater chance of getting them to watch.
Good luck and I look forward to seeing your results!