The Xen Campaigns is a single player episode for Half-Life: Opposing Force. Included are 14 maps, some custom sounds, skins and some custom sentences.
Pretty fun
Total clusterfuck. The firefights are supposed to be huge and epic but the black ops and marines end up killing friendlies by accident due to the awful level design. Just a mess, I'm a diehard OP4 fan and this is one you want to stay away from.