Nihilanth

The Nihilanth is the final boss of Half-Life, and the leader of all the monsters from Xen that have been invading the Black Mesa laboratory after Gordon Freeman accidentally opens up a portal to another dimension at the start of the game. It is a huge, hideous corpse-like creature with an enormous cranium, a body structure resembling a human fetus, a third arm protruding from its chest and what appears to be a mechanical device attached to its underside allowing it to float. It can be heard towards the end of the game uttering cryptic, disturbing messages to the player telekinetically.

The Nihilanth resides in a tall, circular organic chamber with orange crystals which it draws power from. It attacks Gordon Freeman by shooting energy spheres at him, and also by teleporting him away into another room with more enemies in it. Gordon defeats the creature by destroying the energy crystals that protect it from damage, exposing its vulnerable brain. He then jumps on top of its head and destroys the brain, causing the Nihilanth to explode.