Forlorn Dimension by M.T. Satterlee

After getting separated from her family during their hunt for an especially deadly monster, Ellen sets out in search of the Monad Fortress, the safest and most peaceful place in the world. And she finds it, just in time to learn that it’s about to be destroyed.

The sky over the fortress is turning black and monsters are beginning to appear in every corner of the dimension. Saving the fortress is impossible. Even saving herself and her new friends is going to be a miserable struggle that she might not survive.