The Crossing is an upcoming first person shooter computer game, under development at Arkane Studios with assistance from Valve Corporation, fusing single and multi-player gameplay.

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 will take the player across both universes.

Gameplay
The Crossing attempts to fuse single player and multiplayer gaming, allowing external players to take the place of enemies in the single player campaign. To the players in multiplayer mode, they may be trying to avoid a more powerful character or must assassinate him (the single player). 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.

Arkane-Studios.com - The Crossing[forum.arkane-studios.com]
The Crossing Wiki Page[en.wikipedia.org]