Half-Life/Enemies

Barnacle
Barnacles are stationary aliens that attach to ceilings. They have extremely long and sticky tongues that they let hang down from where they hide; if something gets stuck to the tongue, they immediately begin reeling it in, hoping to get another meal.

Headcrab
Headcrabs are your basic alien enemy. They're weak, but also very small, making them hard to hit. Headcrabs are usually found hiding in damp or dark areas, waiting for prey to ambush. When attacking, they launch themselves at their intended victim's head, making a screeching sound. While this would cause trouble for most other humans, Gordon's HEV suit protects his head.

Zombie
Zombies are humans that have fallen victim to headcrabs. The infestation has mutated their hands into long, sharp claws, and there is a giant gaping hole in their torsos. Zombies are usually found standing around doing nothing, but are sometimes found on the floor, feigning death until they are shot or someone approaches. To attack, zombies slash with their claws.

Hazardous Environment Combat Unit
Also known as grunts, marines, or simply the military, the HECU are a special military task force dispatched by the U.S. Government to contain the incident at Black Mesa by killing everything, including the facility personnel. HECU soldiers are the most common human enemy, and utilise a variety of weapons, vehicles and tactics against you. Grunts are usually armed with an MP5 submachine gun and hand grenades, and will utilize squad tactics to surround and destroy you. In many instances, the soldiers also make use of vehicles, such as tanks and attack helicopters, and fixed weapons and fortifications, such as mounted .50 caliber machine guns and artillery pieces.

Assassins
Commonly known as Black Ops, Assassins are elite female operatives skillful in stealth and close combat, and utilize silenced pistols, grenades and (when up close) a variety of martial arts kicks and jabs. They have been confirmed to work independently of the HECU and apparently have their own objectives to complete, one of which is eliminating you. Black ops are extremely fast and agile and can leap considerable distances, making them very hard to target. Often the only valid tactic is to back into a corner so they can only come at you from two directions which you can more easily monitor. If you have any, snarks are extremely effective against assassins, even if they don't kill them outright. When an assassin encounters snarks they will usually ignore you to fire wildly at the creatures, giving you a clear shot.

Gonarch
A giant beast that looks similar to a spider, Gonarch is a four legged creature with a large sack (similar to an abdomen) that hangs down below where its legs meet. As the second to last boss, Gonarch can only be found once during the game and it takes a large amount of damage to kill. The sack is Gonarch's weak point, and it can only be harmed when Gonarch isn't in "cinematic mode" - in which the beast progresses farther into its lair between battles.

Nihilanth
The boss of the game.