Alien Horde Co-Op Mode could be Infinity Ward’s answer to Treyarch’s popular Zombies game mode, featured in previous CoD: Black Ops and World at War titles.
A few days ago, a new Call of Duty: Ghosts trailer allowed us to see an unannounced game mode called “Extinction”. You can read the ‘hidden text’ at the 0:24 mark in the background, just in the top right-hand corner: “Play Extinction Online”
Of course, this game mode has not been detailed, but a leaked listing of the game’s 9 secret Achievements/Trophies spotted on Exophase give us some clues about it. The list shows achievements like “Complete all Extinction challenges and escape”, “Escape with all four players” and “Escape 1st time”, which suggest some sort of survival co-op mode where players have to survive against unknown enemies, complete challenges or escape the situation while possibly under a time limit.
We don’t know the kind of enemies the players may be facing, but if we look carefully at the logo that appears clearly on the “Completionist” achievement, you can see the shape of what appears to be an ‘alien’ skull.