The headcrab is the main protagonist of Half-Life Zombie Edition. He is unleashed upon Black Mesa, after the Resonance Cascade.
In his headcrab form, he is mainly weak, but can go into vent shafts due to his small size.
When he "possesses" someone, transforming the poor guy into a headcrab zombie, he has a wide range of abilities; for example, launching other headcrabs; feeding on corpses to gain health; and he can also launch barnacles. He also has a unique range of mutations: Crusher, Breeder, or Rusher, each with their own special capabilities.
Throughout the game, he travels through Black Mesa, escaping or killing HECU and Barney guards, feeding and possessing scientists, until the detonation of the nuke; after that, he briefly sees the G-Man, before seemingly being obliterated in the explosion.