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The Crossing is a first-person shooter video game by Arkane Studios, which attempts to fuse singleplayer and multiplayer by threading its singleplayer campaign through live multiplayer games.
The game was put "on hold" in May 2009 after the company ran into "an unexpected financial challenge" and decided to focus on smaller projects. 
The game incorporates the idea of parallel universes, and is set across two vastly different renditions of modern day Paris. In one, which shares many aesthetic similarities to our own, Paris has descended into anarchy following the collapse of government. In the other, the timeline diverged in 1307; where instead of being disbanded, the Knights Templar seize control of the French crown. The story takes the player across both universes.
The Crossing attempts to fuse single player and multiplayer gaming, allowing human players to take the place of enemies in the single player campaign. The players who take on the role of AI grunts play a team-based deathmatch game, with the additional goal of trying to kill or protect the one or two more powerful players who are playing through the story. This amalgamation has been dubbed "crossplayer" by Arkane.
The game also includes a team-based multiplayer mode called skirmish, which too, is incorporated into the single player campaign, where the main player might try to avoid the skirmish.