In Half-Life for PlayStation 2, the players are able to play the game as a Vortigaunt in two game modes.
When controlling a Vortigaunt, the player cannot pick up any weapons or items. Health can be regained by damaging enemies and breakable objects; this further shows that Vortigaunts can drain life force from their enemies. Health is displayed as a bar on the bottom of the screen. It is much easier for the player to die due to the lack of HEV suit protection. The Vortigaunts are faster than human player characters, have a larger field of view (similar to the fisheye lens effect), and can attack their enemies in two ways: by using their claws or by firing a green, lightning-like energy beam that requires a vulnerable period of "charging-up" before being unleashed. When playing as a Vortigaunt, the player character can be heard making idle sounds. The players bleed yellow blood like other Vortigaunts.
The gameplay is the same as Xen Attacks with several differences. The in-game dialogue is replaced with gibberish sound effects, making it impossible to understand lines spoken by the human characters. The player can pick up the HEV suit, which is required to advance in the game, and use its flashlight. The suit chargers cannot be used, and the health chargers hurt the player when used. Unlike other aliens, Barnacles ignore the player in the Alien Mode. The game ends when the player reaches Xen in a special ending.
The gameplay seems to be inspired by the Alien mode in Rebellion's Aliens versus Predator (1999). As an Alien, the player must survive by killing and eating the enemies to regain health. The health bar looks and works the same way. The field of view is wider than the vision of other playable characters.
Overview
Xen Attacks
In Half-Life: Decay, there is a bonus mission where the players assume the role of two Vortigaunts, Drone Subjects X-8973 and R-4913, who are sent to Earth by the Nihilanth to retrieve the crystals that were stolen from Xen by the Black Mesa Science Team, who are using the crystals in their research. This mission can be unlocked by getting an A-grade rating on all missions.When controlling a Vortigaunt, the player cannot pick up any weapons or items. Health can be regained by damaging enemies and breakable objects; this further shows that Vortigaunts can drain life force from their enemies. Health is displayed as a bar on the bottom of the screen. It is much easier for the player to die due to the lack of HEV suit protection. The Vortigaunts are faster than human player characters, have a larger field of view (similar to the fisheye lens effect), and can attack their enemies in two ways: by using their claws or by firing a green, lightning-like energy beam that requires a vulnerable period of "charging-up" before being unleashed. When playing as a Vortigaunt, the player character can be heard making idle sounds. The players bleed yellow blood like other Vortigaunts.
Alien Mode
The Alien Mode allows the player to play through Half-Life as a Vortigaunt. This mode can be activated from the Cheat Codes menu in the Options, either by getting an A-grade rating on all Half-Life: Decay missions, including the bonus mission, or entering a code. The code is: Up, Triangle, Up, Triangle, Up, Triangle, Up, Triangle.The gameplay is the same as Xen Attacks with several differences. The in-game dialogue is replaced with gibberish sound effects, making it impossible to understand lines spoken by the human characters. The player can pick up the HEV suit, which is required to advance in the game, and use its flashlight. The suit chargers cannot be used, and the health chargers hurt the player when used. Unlike other aliens, Barnacles ignore the player in the Alien Mode. The game ends when the player reaches Xen in a special ending.
Behind the scenes
A cheat code can be found in one of the areas in Xen Attacks. The code is: "Up, Triangle, Up, Triangle, Up, Triangle, Up, Triangle, Right", similar to the code for the Alien Mode. When entered, however, the game says it is invalid.The gameplay seems to be inspired by the Alien mode in Rebellion's Aliens versus Predator (1999). As an Alien, the player must survive by killing and eating the enemies to regain health. The health bar looks and works the same way. The field of view is wider than the vision of other playable characters.
Gallery
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Viewmodel. 
The HUD.
Charging.
Electrical attack.