Servants of the Crossed Arrows — Ginn Hale

Volume Two. Environmental Studies graduate student John Toffler intercepts
a letter intended for his mysterious roommate, Kyle. Inside
John finds a key and a note reading: don't. Before he can return
the key to Kyle, John and his friends, Laurie and Bill, accidently
use it to open a gate to another world. In an instant they are
torn from the easy life they've known and hurtled into the harsh
wastelands of Basawar.
Kyle -- who in his own world is known as Kahlil -- realizes
that he must kill John before he destroys Basawar. Despite the
Great Gate being shattered, he follows John, Laurie and Bill back
to his homeworld. But the passage through leaves him physically
beaten and his memories broken. Confused and alone in a world
he no longer recognizes, Kahlil takes refuge among men who
have a use for his deadly skills.