The Core is a total conversion single-player mod for Half-Life. Survive the disaster as Maurice Andrews, a low level engineer stationed in Black Mesa during the events of the resonance cascade. Seemingly trapped, Andrews must fight his way to the surface to survive.
By the time Andrews reaches the central complex dormitories, the place is infested. The small, cramped living quarters of the low level science personnel have become a series of tight, frantic combat zones.
The grunts, now left behind by their retreating comrades, are fighting a losing battle as the Xen forces pour into the facility in greater numbers, annihilating everything in their path. Meanwhile, science team members seeking refuge in their dorms are falling victim to the large number of headcrabs that are now themselves scurrying into the relative safety of the lower levels to nest.