



The in-game dialogue is replaced with gibberish sound effects, making it impossible to understand lines spoken by the human characters. The gameplay is the same as Xen Attacks with several differences. The code is: Up, Triangle, Up, Triangle, Up, Triangle, Up, Triangle. 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 Alien Mode allows the player to play through Half-Life as a Vortigaunt. The players bleed yellow blood like other Vortigaunts. When playing as a Vortigaunt, the player character can be heard making idle sounds. 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. It is much easier for the player to die due to the lack of HEV suit protection. Health is displayed as a bar on the bottom of the screen. Health can be regained by damaging enemies and breakable objects this further shows that Vortigaunts can drain life force from their enemies. When controlling a Vortigaunt, the player cannot pick up any weapons or items.

This mission can be unlocked by getting an A-grade rating on all missions. 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.
