Is Sauron able to play a shadow card before the final combat in the finale? We looked through the rules a few times and didn't find anything that says he can't, but we wanted to be sure since it made such a big difference in the outcome. Specifically, he played "The Black Breath" which says "Play at the start of a combat involving a minion". It gives the hero a corruption, -1 strength, and -1 agility in the battle for each corruption card he has. In our case this gave the hero a -4 penalty, and as you can image the Ring Wraiths dispatched him pretty handily.
I tend to think we played it correctly and this is a lesson in why heros should think about avoiding and removing corruption cards when they can. In the game we played none of the heros ever removed any of the corruption cards they received and didn't really think twice about picking them up when they were given a choice.