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.
Ugh, its one of those mods. Those ones that ramp up the damage by 5x and then add those fast moving headcrabs, it also uses freezecrabs which blind you for 10 seconds. Honestly I'm not a fan of ultra hard realizm mods because well it artificially lengthens the game by making the player die alot. Although I guess it kind of means you get to use those headcrab dodging skills to the maximum breaking point. All I can say is it worth it for the level design? No. Its fairly standard stuff, using opposing force weapons and a couple of gunman chronicle models.