The Iron Banner as we know it is no more. Essentially, it's still about excelling in combat and proving you are an exceptional guardian. But few substantial novelties have been made:
- Battles are now 4×4 instead of 5×5;
- Level advantages are now irrelevant (just like in Crucible overall), but your skills will determine the outcome;
- Bounties and Ranks are now replaced by Iron Banner Engrams — Lord Saladin in the Tower is the guy to ask;
- The familiar Tokens system works here: you earn them anyway, but winning (or special Ghost shells) gets you more;
- There are Daily and Season Milestones to track your progress;
- An eye-candy set of a gear for all classes. No updates on weapons yet.
All you need to do to enter the event is to a) finish the main campaign and b) find Lord Saladin in the Tower to receive the Iron Banner Quest. And of course, tune in the game starting October, 10th 2 AM PT.
In spite of squeezing in the word 'Season', Bungie did not provide details in their blog post. This concept will presumably be explained at their Twitchcon panel discussion on October, 20th 4:30PM Pacific. You can virtually join it via the convention's Twitch (of course) channel.
As to the Prestige Leviathan raid, it arrives on the same day as the Iron Banner at 10 AM Pacific. It requires 300+ Power and promises more intelligent enemies and new mechanics. In a very encouraging tone, Bungie say they do not 'expect everyone to be successful in completing this activity'.