History VideoGame / HalfLifeDecay

The game requires the players to cooperate in order to solve puzzles and complete tasks, with each player being [[GameplayGrading ranked at the end of each stage]] depending on weapon accuracy, enemies killed, and damage taken. ''Decay'' was released exclusively for the UsefulNotes/PlayStation2 and came bundled as part of the port of first ''Half-Life''. As such, it's the only game in the franchise to have not seen an official release on the PC. However, dedicated fans were able to port the game to Windows as a GameMod and added the ability to support online multiplayer.

* DeathCourse: The third mission requires traversing a heavily damaged version of the Hazard Course to fix an elevator for Dr. Rosenberg. Pieces of the course are broken or missing, the holograms glitch out, and some passages are blocked altogether.
* DistressCall: The fourth mission involves setting up a satellite dish on the surface, to send an emergency signal that calls in the military.
* DoctorDoctorDoctor: One of the first security guards Collete Green and Gina Cross meet on their way to work greets them with "Good morning doctor, ''[beat]'' doctor."
* EasterEgg:
** At the very start of the chapter/mission "Intensity", if you shoot the keypad before Dr. Richard Keller starts briefing the players, you get a short and humorous piece of dialogue.
--->'''Dr. Keller:''' Good job for doing previous stuff. Now let's get ready to do new stuff.
** The fanmade PC port includes a bonus puzzle in the chapter "Domestic Violence", involving a pair of codes to be entered on a hidden UsefulNotes/{{P|layStation2}}S2 controller that unlocks the secret "Aliens" mission.
* FailsafeFailure: The Dampening fields surrounding the Anti-Mass Spectrometer, which would have prevented the resonance cascade had [[spoiler: the G-man not shut them down]]. The "Resonance" mission centers on resetting these fields, but when Drs. Cross and Green do, Dr. Keller finds out that [[spoiler: the Nihilanth is keeping the dimensional rift open.]]

* LevelInReverse: The final mission is an inverse of the last level of ''Half-Life'', where Gina and Collette have to protect the Resonance Beacon from waves of Xen aliens, while charging three huge Xen crystals to finally close the dimensional rift between Earth and Xen.
* LightAndMirrorsPuzzle: The chapter "Intensity" involves a laser-and-mirrors puzzle that powers the Resonance Beacon.

* UniqueEnemy: The final level is the only time in the whole series where you actually fight one of the Alien Aircraft (nicknamed "Manta Rays" by fans). They do appear frequently in the other three Black Mesa era games, but only in the background, in scripted sequences, or as props, not as enemies.

* OffModel: When playing co-op on [=PS2=], only the crowbar's view model is shown on-screen, but when swinging or playing solo where all of the view models are shown, you can clearly see how the view model hands are just Gordon's orange hands from the main game, despite the fact that Colette and Gina are wearing maroon and tan HEV Suits.

** At the very start of the chapter/mission "Intensity", if you shoot the keypad before Dr. Richard Keller starts briefing the player´s, you get a short and humorous piece of dialogue.

* OffModel: When playing co-op on [=PS2=], only the Crowbar's view model is shown on-screen, but when swinging or playing Solo where all of the view models are shown, you can clearly see how the view model hands are just Gordon's Orange Hands from the main game, despite the fact that Colette and Gina are wearing Maroon and Tan HEV Suits.

* OffModel: When playing Co-op on PS2, only the Crowbar's view model is shown on-screen, but when swinging or playing Solo where all of the view models are shown, you can clearly see how the view model hands are just Gordon's Orange Hands from the main game, despite the fact that Colette and Gina are wearing Maroon and Tan HEV Suits.

* DummiedOut: There's unused voice lines indicating that in SP you'd be able to ask your AI partner to follow you/hold position, in the [=PS2=] game you have to swap between the characters to move them.

''Half-Life: Decay'' is the third and final ExpansionPack to [[Creator/ValveCorporation Valve's]] ''VideoGame/HalfLife1'' developed by Creator/GearboxSoftware. The game is unique in the series for exclusively featuring CoOpMultiplayer, with Single Player being an afterthought.

** The bonus mission takes place in the same parking garage from the end of ''VideoGame/HalfLifeOpposingForce'', including [[spoiler: the truck where the Black Ops' nuke would be set off.]]

* DummiedOut: There's unused voice lines indicating that in SP you'd be able to ask your AI partner to follow you/hold position, in the PS2 game you have to swap between the characters to move them.
* EasterEgg:

--->'''Dr. Keller''': Good job for doing previous stuff. Now let's get ready to do new stuff.
** The fanmade PC port includes a bonus puzzle in the chapter "Domestic Violence", involving a pair of codes to be entered on a hidden [[UsefulNotes/Playstation2 PS2]] controller that unlocks the secret "Aliens" mission.

** At the very start of the chapter/mission "Intensity", if you shoot the keypad before Dr. Richard Keller starts briefing the player(s, you get a short and humorous piece of dialogue.

* GameMod: The game was ported to PC by dedicated fans, with the ability to switch between the two characters (and basic A.I. for the non-player-controlled character) added to allow the game to be played in single player.

''Half-Life: Decay'' is the third and final ExpansionPack to [[Creator/ValveCorporation Valve's]] ''VideoGame/HalfLife1'' developed by Creator/GearboxSoftware. The game is unique in the series for exclusively featuring CoOpMultiplayer, with no option for single-player.

''Half-Life: Decay'' is the third and final ExpansionPack to [[Creator/ValveSoftware Valve's]] ''VideoGame/HalfLife1'' developed by Creator/GearboxSoftware. The game is unique in the series for exclusively featuring CoOpMultiplayer, with no option for single-player.

* ContinuityNod: The mission "Domestic Violence" takes place in Level 3 Dormitories, and you can find Gordon Freeman's room in one of its wings.

* LightAndMirrorsPuzzle: The chapter "Intensity" involves a laser-and-angles puzzle that powers the Resonance Beacon.

* AscendedExtra: Dr. Rosenberg from ''VideoGame/HalfLifeBlueShift'' appears at the beginning, and he accompanies you in two of the missions.
* DistressCall: The fourth mission involves setting up a satellite dish near the Hazard Course, to send an emergency signal that calls in the military.

* AscendedExtra: Dr. Rosenberg from ''VideoGame/HalfLifeBlueShift''

* LightAndMirrorsPuzzle: The chapter "Intensity" involves a laser-and-angles puzzle that powers the Beam Matrix.

* EasterEgg: At the very start of the chapter/mission "Intensity", if you shoot the keypad before Dr. Richard Keller starts briefing the player(s, you get a short and humorous piece of dialogue.
-->'''Dr. Keller''': Good job for doing previous stuff. Now let's get ready to do new stuff.

%%* TeamworkPuzzleGame

The game requires the players to cooperate in order to solve puzzles and complete tasks, with each player being [[GameplayGrading ranked at the end of each stage]] depending on weapon accuracy, enemies killed, and damage taken. ''Decay'' was released exclusively for the ''UsefulNotes/PlayStation2'' and came bundled as part of the port of first ''Half-Life''. As such, it's the only game in the franchise to have not seen an official release on the PC. However, dedicated fans were able to port the game to Windows as a GameMod and added the ability to support online multiplayer.