In a vast cave, Gordon finally confronts the powerful and mysterious being that is holding the portal open. Gordon destroys the Nihilanth’s shield, allowing him to destroy its vulnerable brain. As the creature dies, it floats toward the ceiling, and explodes in a giant green blast that overpowers Gordon’s senses.

“It’s time to choose!”
– The G-Man

Gordon’s journey across Xen continues, but his supplies are running short, spent on destroying the Gonarch.

He also comes across the Xen army’s Alien Grunt cloning facility, where the army is created.

It can be noted that the Vortigaunts in one of the factories are not hostile towards the player, and will ignore him unless attacked.

Freeman is made to face one of the Nihilanth’s most powerful minions: the Gonarch in her lair.

You’ll have to fight her in 3 different stages, but there is health available if you look carefully.

Gordon Freeman arrives in the borderworld, Xen.

On the strange borderworld, Gordon encounters many of the aliens that had been teleported into Black Mesa in their natural habitats, as well as the remains of HEV-wearing researchers that came before him.

In this chapter, Gordon arrives at the Lambda Complex, where he believed the science team to be sheltering from the Xenians, only to find that the majority of the complex is now overrun by hostile creatures. He is informed that he must flood the reactor chambers in order to access the core, and that a team of scientists is waiting for him at the tip of the reactor.

In this chapter, the HECU determine that they’re fighting a losing battle against the Xen aliens and, having continually failed to neutralize Freeman, abandon their mission and begin to evacuate the facility, calling in air strikes and heavy artillery to cover their retreat. Meanwhile, Gordon makes his way into an underground storage garage to avoid the bombardments, and eventually fights his way through scores of aliens and several groups of the decimated HECU forces before reaching the Lambda Complex.
The Marines begin to evacuate Black Mesa and air strikes begin. Gordon navigates underground water channels and tries to avoid scores of alien invaders as they pick off remaining Marine stragglers.

After arming himself with advanced prototype weaponry, Gordon again reaches the surface. It has become a war zone. The Vortigaunts, along with Alien Grunts and Gargantuas, have begun making determined attacks against the Marines, and the tide of the battle is beginning to turn in favor of the aliens.

The Marines call in reinforcements, but it isn’t enough. Gordon must scale cliffs and navigate the bombed out buildings while avoiding both sides as the forces of Xen begin to dominate the battlefield. Finally, Gordon reaches relative safety underground.