The Lara Croft doppleganger first appeared during the main story of Tomb Raider: Underworld, created to best Lara at her own game. Of course the creature fails, but what if it didn't? Apparently we'd be playing a Prince of Persia game. The doppleganger is a melee fighting machine, running, jumping, and beating the hell out of everything in her path. It certainly looks a lot more entertaining than standard Tomb Raider fare, mainly because they've taken Lara's model, swapped abilities, and created a whole new way to play the game.
The Xbox 360-exclusive Lara's Shadow downloadable content is currently scheduled for a March 10th release.