Black Mesa: Elimination
Elimination (also mentioned as Survival and Coop in the game files) is a PvE cooperative survival multiplayer game mode available in 
Black Mesa. It is an unfinished game mode in which players must defend the base from zombies throughout whole round. It is the only official cooperative PvE game mode. Unlike other official game modes, players are not divided into opposing sides, they are all in single team (used info_player_deathmatch entity as a spawn point).
The Main Goal
To finish match, you need to kill all the zombies that appears in groups of waves. Players can respawn without limitations, so in order not to lose, you also need to prevent the zombies from destroying the player base.
Player Base
It is base that spawns at map center. Maybe it is the entity that also have been able to upgrade players weapons, give heathkits and other stuff by interacting with this entity, as said by Adam Bomb here.
In the current stage of the game mode, it uses bottom part of the anti-mass spectrometer as a placeholder model, the size of the model is reduced by half. This entity it doesn't receive any damage, so it's not possible to lose in this game mode.
For more information, see the page about this entity.
Zombies
The zombies is NextBot's. This entity is not related to NPC zombie version from singleplayer campaign, their AI behavior and navigation are completely different. They're using navigation mesh to spawn and move. Usually zombies appear in areas of a map that are not visible to the players (one zombie at one point), but sometimes something goes wrong and one or more zombies may appear at points that are visible for players.
For more information, see the page about this entity.
Director
There are also a director entity. Probably an Artificial Intelligence that controls some events and the appearance of zombies, similar to the director from 
Left 4 Dead.
For more information, see the page about this entity.
Gameplay
After the warm up time, the player base and director appears at map center. At the same time, the zombie spawn mechanism is launched with the counter of waves and counter of killed zombies. By default, there are 15 waves, 32 zombies in each wave, maximum active zombies is 8 (due to this you will not be overwhelmed by the quantity). After killing 32 zombies, you are given a short break before the next wave. After the last wave, the game will finish round and show the leaderboards (it's worth to note that you don't get points for kills, with exception of suicide). Due to a bug, the game will not launch the next map.
Throughout the game, the waves and the zombies themselves do not become more difficult, which makes the gameplay feel extremely monotonous. You also can't interact with the base.
All of this is a rough sketch that was never completed.
Development
There is almost nothing known about the development process. It was made by programmer Lucas, the mode itself is not present in version 0.0.0, which means that it appeared later than the main multiplayer game modes.
The only official map there you can play this game mode is dm_crossfire, the only map with functional navigation mesh.
On December 13 2020, this mention of game mode in the game files was found and mentioned by CitizenFive in the page about some cut content (this edit).
On January 26 2024, the way to activate it was found by MyGamepedia randomly, then was tested many related stuff.
At present, the developers do not mention this mode and do not make any changes for it. Most likely, it will never be finished.
Dedicated Console Variables
| Cvar/Command | Parameters or default value | Descriptor | Effect |
|---|---|---|---|
mp_coop | 0 | Bool | The cvar that allows you enable/disable this game mode. 0 - disabled, 1 - enabled. |
survival_nextwavetime | 15 | Integer | The amount of seconds before next wave. |
survival_wavecount | 15 | Integer | The amount waves. |
survival_sceneselectionfile | survival_scenes.txt | String | Unknown scene selection file. This file is not in the game files. Todo: Find out if this file is present in older versions of the game. |
Bugs and Limitations
Bug:This game mode is unfinished and has several bugs and issues:
- The game will not launch the next map from mapcycle.txt after last wave (
changelevel_nextdoesn't work). - Server shuts down when you launch a map with the console command.
- You can't lose in this game mode (the player base can't die at all).
- Snarks don't attack zombies, instead they attack you and other players.
- The announcer works in the same way as in deathmatch game mode.
See Also
- bot_zombie - the bot zombie entity.
- director_resource - the director.
- entity_player_base - the base.