Despite claiming there would be no downloadable content to add a Mission Mode to Ninja Gaiden II, Team Ninja created just such DLC. Available for just a short while before being taken off Xbox Live Marketplace, the new DLC will run you 800 Microsoft Points ($10) and comes with 24 arena-style missions.
The first 16 missions, called Karma Attack, give you Ryu Hayabusa's full arsenal of weapons, each fully upgraded. You can choose whatever weapon and ninpo you prefer and take on a set number of enemies in a section from the retail game. Defeating the enemies isn't so tough, especially if you were good enough to beat Ninja Gaiden II. The challenge is in maxing out your karma to earn a high score and get a good ranking in the online leaderboards.