SWARM: The Griffins' War by Vicent Casil

Moran is a soldier from a distant future, and he and his men have a war against a mass of mythical creatures to save their metropolis. But their only hope lives in the past. It’s 1969, and two college friends from Berkeley, David and Nathan, plan to visit one another’s state for homecoming. First, they visited Michigan, where Nathan lives, introducing his cousin Raymond. The three went duck hunting and were oblivious to their journey of entering a portal that took them in Moran’s time.