Nihilanth's death stopped any future alien attack on Earth. But after some time, there raised something more evil. It was called Mother of Headcrabs. This monster started creating more powerful headcrabs, than anyone could imagine. There was a lot of varieties and different specialities. They were true warriors, really strong enemies... - By Michal Jirouš.
I would've enjoyed it if there weren't so many headcrabs with such crippling effects. I had to turn on god mod after the first headcrabs because I couldn't kill it before it destroyed me.