You was hunting in the woods and then this strange fog raised up. You grabbed your shotgun when you heard a voice outside. Wtf Zombies. Try to stay alive for 5 minutes and kill them all to win and unlock next difficulty level.

(FPS arena wave shooter)

In the last update the endless mode was changed to difficulty levels (easy, medium, hard, very hard, hell on earth, impossible). You will start in easy mode and have to survive 5 minutes and kill all so far spawned enemies to unlock next difficulty level.

The 2 player pve coop multiplayer mode is hard level endless mode.