I think this might get more takers if the fights were not to the death. At least regarding player characters. Perhaps there is some magical safeguard in place that prevents characters from actually being killed. That's not to say they can't and won't get injured though.
I could offer up my character Nemeril for this, she is an excellent archer and would likely be interested. While she does have some melee skills, she is far better at ranged combat. Perhaps divide tournaments up into melee versus ranged?
While in the background, a dark secret is brewing by none other than the Grand Master himself. Maybe he doesn't want anyone to leave the island and only fight for the rest of their lives solely for his own entertainment? Then characters can investigate after enough suspicions have been raised.
I also think it would be neat if the Grand Master himself was never seen upfront, only through his various eyes and ears (aka assistants and minions of his) that inform participants? As added suspense, a mysterious face could peek out from behind the curtains way up in his grand estate. The Grand Master himself. 8)
