Spear And Rifle (canceled sorta)

A mysterious creature bursts out of a mountain and kills thousands US citizens with it's mere word, then crawls back into the hole from which it came. The military tracks down the beast only to find another world at the end of the tunnel, one with magic, monsters and medieval armies which can be shockingly adaptable against more advanced enemies.

(Notice: I will no longer be writing this book in favor of writing another one which will take place in a different perspective, the events of this book is still kinda cannon.)