Obsidian Conflict is a cooperative multiplayer modification set in the Half-Life 2 universe.
It features a large amount of new gameplay features on all-new maps specially designed for the mod with focus on cooperative gameplay, as well as over hundred user-made maps following the same formula.

In addition to that, you're able to play through vanilla Half-Life: Source all the way up to Episode 2.

Key features

• All Source engine Half-Life series games playable in co-op online (Half-Life: Source, Half-Life 2, Lost Coast, Episodes 1 & 2).
  
• A large set of included custom maps with a great variety of gameplay styles, from classic walkthrough combat and deathmatch, to teamwork puzzles and lives mode, or even abstract game modes like fishing or farming.
  
• New weapons, including dual Uzis, Medkit, Sniper Rifle, the Alyxgun, the Tau Cannon, deployable Manhacks, as well as support for scripted weapons allowing use of weapons from Counter-Strike and Day of Defeat Source.
  
• New NPCs, like the Mortar Synth and support for NPCs from all the Half-Life games like the Gonarch or the Hunter.
  
• Many co-op related features, such as lives system, HUD waypoints, player locator, attention & medic calls, merchants, score as well as players' inventory carrying over between level transitions.
  
• Great support for community mappers, including features like the mapadd system to easily edit existing maps without having to recompile them, as well as tutorials and prefab examples for all of OC's new entities and features.