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.
Not bad, if you try it on easy difficulty. But on hard some places are really pain in *** :0