On the general forum I heard you mention that adding a NPC caused it to black screen. While making the colliseum I encountered the same problem, I just want to make sure that you realize it's not adding a character that was the problem it was the character that you added. For example if I loaded the second version of speekio for the second fight, loading the 3rd version of speekio for the third fight would cause a black screen. On the same note if I loaded the 4th version of speekio for the third fight everything was fine.
I can't find any pattern to this very annoying problem and can only suggest trial and error until you find the right guy. For some reason the programmer's of CT made it so if enemy A is loaded Enemy B can't be loaded.
--jp
