Half-Life: Decay

Half-Life: Decay is perhaps the most obscure official content in the Half-Life franchise. This expansion pack was released only for the PlayStation 2. A PC version was apparently planned, but never got released due to unknown reasons. The game requires two players, as it follows two scientists at once: Gina Cross (the woman seen in the Black Mesa training course) and Colette Green. An unofficial PC port was made and released in 2008, adding online multiplayer and a primitive bot for offline play.

Decay contains a bonus mission through the perspective of two vortigaunts, simply referred to as X-8973 and R-4913. These two vortigaunts are tasked with retrieving stolen crystals from Black Mesa.

A Gearbox employee once noted on an internet forum:
 * ''I work for Gearbox and did most of the coding for the Decay work we did on the PS2. First I'd like to say that there is no interest or desire on our part to fox this project. Everyone here thinks it would be great to see this ported to Source. We even had a port of the game ready on HL1 for the PC, but due to powers beyond our control, were unable to release it at the time. So it's always been of interest to us to see Decay make it to the PC since, as someone else here has mentioned, it does deliver a part of the Half-Life storyline so it's always been a shame to prevent a large portion of the Half-Life players out there from seeing that part of the story. When this mod is done I will probably fire it up with a friend of mine that never had the chance to play Half-Life on the PS2. It's been a while since I played through Decay (though I certainly played through it quite a bit while we were working on it).
 * -Patrick Deupree [Originally posted on the VERC]

The PC port is not perfectly accurate to the original PS2 game, and there's evidence of at least minor discrepancies existing. Other ports of Decay have been attempted, but so far only the one has released. Any technical information about Half-Life: Decay on Source Entities refers to the 2008 PC port unless otherwise stated.