The Legendary Arena is the eight and last Arena to be unlocked. You need at least 3000 trophies in order to play in this arena. It doesnt unlock cards since the last ones were unlocket in the Royal Arena. Here is where the Legends play.
The Legendary Arena is unique in that it uses a new type of trophy known as Legend Trophies.
Legend Trophies are gained at the end of every Legendary Arena Season. Each season lasts 14 days. After a Legendary Arena Season has ended, the players trophy count will be reset to 3,000 Trophies. The Trophies that were deducted will be added to the players Legend Trophy count. Legend Trophies do not count towards your ranking in the Leaderboard.