I'm betting arena is going to be like this: every 10 rounds there is a checkpoint which allows you to leave and than comeback whenever you wish. After that checkpoint round is defeated you are rewarded with RNG loot. Every 15 rounds there is a mini boss that spawns which gives higher chances at armor and weapons. By the 60th round( after fighting through 3 boss levels and 6 checkpoint lvls) the main boss, the Kell of Wolves, spawns and once he is killed he has a chance to drop the arena helmet ( only obtainable from him) and other RNG loot. He also has a 4% chance to drop the lord of wolves shotgun.