A user on Reddit has released a bunch of new info and details on Assassin's Creed: Unity claiming to be a Xbox One "Developmental Build Tester". A new mode (shown in this clip gives you an idea of Rift mode that sees popular assassins (Kenway, Altaïr, Ezio, etc.) return in what appears to be speed challenges where you collect glyphs with a timer on various maps.
As for the accuracy of the report, from what we can see from the videos it would appear as if this is indeed a person with access to the game. We expect Ubisoft to react with a proper announcement of this new mode shortly. As you may recall the game's announcement was forced due to leaks so it would seem keeping this game under wraps is a challenge.