Okay, so I'm writing a story about a girl who lives on earth. She's having a normal swimming lesson (she comes from Belgium, where it's normal that primary-school children have swimming lessons at school) when she finds a weird box that she can't unlock. She ends up going to a magical world (sort-of like Middle earth) where she finds out the box belongs to this evil guy who ruled the world for 2,000 years and the box contains his soul. To defeat the evil guy, she needs to destroy his soul, but to keep it safe, he hid the box in the town swimming pool where my main character takes swimming lessons, whilst keeping the key with him at all times. She needs to get the key to open the box to destroy his soul. What are some clichés I should avoid when writing this type of story?