Occasionally the player will be challenged by the Banished (or get captured by them if they run out of lives), if they accept they can get into a banished phantom that will take them to an arena to fight against challengers or other captured prisoners (Flood, Prometheans, Covenant, Heretics or Insurrectionists) for either bonus rewards from challenges, or to avoid game over (with a cap of once per round) if you get captured by them!
I haven't fully decided on how this system is gonna work yet, but it should be cool!