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 2 all the way up to Episode 2, as well as experimental support for cooperative Half-Life: Source play.
Do you like coop? Did you like Synergy? Then keep reading.
Obsidian Conflict is one of the best coop mods out there, and I'm really sure I can compare this to many coop games out there, because this probably beats most AAA coop titles you play.
This is bigger than Synergy. The devs thought of everything! Obsidian Conflict is a mod in which you can play HL2/EP1/EP2 and even more, HL campaign (didn't test HL though), in a totally messy and fun coop. Even better, this has a huge amount of maps made by both the developers and the communnity, great maps which you will play and replay all over again and still find it fun.
The game also features lots of new weapons for your shooting needs. OC has also a great and incredibly balanced money system for buying weapons from NPCs on most maps.
Well, as you can see it is really hard to describe Obsidian Conflict. I said the basic you should know, if you want more, I recommend you to play the game and discover yourself!