The arena is Gods Rush PVP mode and it unlocks at team level 10. Here you test your might against other players from all over the world for shards, items, gold and gems. All arena battles are set on auto-battle and cannot be user controlled.
In arena you get a list of three random players you can choose to attack. Their name is listed along with their arena rank, team power level, and how many arena credits you will receive for beating them. The defense tab is to set up your team to be used in battle when another player challenges you. In the records tab at the bottom you can view how you have done against other players in battle as well as watch replays of previous battles
On the left side you can see how many arena credits you gain per hour. You can hold a max amount of 200 credits before you need to collect them to gain more. Higher ranking in the arena will net you more credits per hour, more gold and gem rewards and a base amount of arena credits per day. Your entries are listed here. Entries are the amount of times you can attack in the arena. You start off with 5 per day, however you can buy more with gems.
Arena entries reset at 5am EST daily
Arena ranks update at 9pm EST daily. Rewards are then sent via in game mail
The attacking player loses if time runs out
If a player attacks and defeats a higher ranking player, the two players will switch ranks
Arena credits can be redeemed in the arena mall