Gordon Freeman

And the thick of things comes wading out to meet them. The backbone of the Combine's armies was a race of Synth more quickly grown and easily altered than humans, and less difficult to control than the so-called Race X (much of which is currently busy assaulting the humans en masse). They were monstrous, naturally armored four-legged things with very little brain but quite a lot of stamina and aggression to begin with. It took little effort to make them obedient, and then to make them dependent, and then to weaponize them- sharpen the claws and pincers, mount automatic weaponry on their undersides, kit them out with equipment that feeds the EM spectrum directly into their sensorium. Cheap, solid, reliable, and deadly; even the Combine could see that as an advantageous combination.

The humans call them crab synths, and two of them are bearing down on the trolls.