When four children walk through a coat-filled wardrobe in England, they find the land of Narnia, a magical place filled with fantastic creatures and talking animals. An evil witch, who has imprisoned Narnia in ice and snow, deceives and captures one of the children. His brother and sisters must seek help from the great lion Aslan, Narnia’s creator and rightful ruler, to release Narnia and rescue Edmund. But the witch invokes the ancient “deep magic,” and Aslan offers himself as a sacrifice in place of Edmund.
Don't use "the" here for magic I'm not familiar with. Quotations like these, too, are worrisome when you're describing your own ideas/work. I would have said, "But the witch invokes ancient magic, and Aslan is forced to offer himself as a sacrifice in place of Edmund."
The witch kills Aslan on an ancient stone table. Then she and her evil army wage war on the grieving Narnians. The children, armed with gifts from Aslan, become leaders in a desperate, losing battle. But unknown to the witch, Aslan’s sacrifice has unleashed a “deeper magic.” Aslan rises from the dead to defeat the witch and redeem Narnia from its eternal winter.
"Ancient stone table" is maybe unnecessary here. "Narnians?" We haven't been introduced to these people, so maybe you want to say something like "inhabitants of Narnia." Again, don't use those quotation marks. And end this "and redeem Narnia with the help of the children." We like to see power in the hands of the main characters, remember. The victory should be theirs as well.
So a few tips, but good work.