[BookCat]

Not only is the vocab too advanced, but so is the grammar. Try to stick to short, simple sentences. I think even words like "hideous" are too difficult for the age-group. Try to use monosyllabic words where possible. I have a degree in Lit and found the grammar a bit convoluted.
The following is a very basic version of your blurb and may be a little too young in word usage, but you get the idea. By the way, what will happen to the kingdom if the gargoyles perish? Seems a bit of "so what?" at the moment. You need something more gripping imho.

Long ago wizards carved statues of ugly creatures. They brought them to life with a jewel called Heart of Rock. The wizards hoped that these ugly creatures would drive away the Nightmares.
Years later, a brave cobbler must look for the special jewel called Heart of Rock so that he can save his world. The ugly creatures will die if he takes away their magic jewel. If they die something bad will happen. (The Nightmares will take over?)