Ascendance is a short story that bridges the gap between AC2 and Brotherhood, which is out this Friday. The blurb reads: follows Assassin Ezio Auditore as he struggles to remove the treacherous Borgia family from their papal throne. Ezio will learn the brutal truth behind Ceasare Borgia's rise to power. Fratricide, betrayal, torture... Cesare employs any tactic to achieve his ambition - ultimate control of Italy."
You can check out a short teaser trailer for it here. It costs $1.99 on iTunes, or 160 MS Points on Xbox.