Unlike humans, the Strogg aren’t just one race. They represent a combination of captured and processed races; a bloody marriage between bone and metal, flesh and machine.
—Quake II manual
The enemies in Quake II are the Strogg - a vicious cybernetic race with mysterious origins. They are composed of various assimilated races merged with advanced machinery. A few specimens from the planet's heavily mutated (and carnivorous) fauna are also among the monsters.